Issue: #681468 by dereine: Add pluggable display extenders.

This allows to define new options/UI elements for the view settings.
It's kind of a horizontal scaling model for views.

commit 15cdbd0ac107f9d7eb11d8214905cd3fe262c22d
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 11 20:58:27 2011 +0200

    fix possible wsod on clear cache

commit bd370f458643b907d13dbc67f8be341084dcc6fd
Merge: 554cf56 a043cb32
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 11 20:39:19 2011 +0200

    Merge branch '7.x-3.x' into 681468-display-extend

commit 554cf566d4a372bdcb46a04b6f3d9d707940ad48
Merge: 2d1dcd6 84640688
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri May 6 22:34:24 2011 +0200

    Merge branch '7.x-3.x' into 681468-display-extend

commit 2d1dcd63224cd890288275a6601daa34414ecde3
Merge: ae32031 73209932
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Thu May 5 20:34:13 2011 +0200

    Merge branch '7.x-3.x' into 681468-display-extend

commit ae3203109131be7df1c335b5f8145640c209129a
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 4 23:32:59 2011 +0200

    fix typo

commit d173248e08947555f120b7e1b52cf2e77beed397
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 4 22:33:42 2011 +0200

    fix codestyle

commit 42598f3361ae3c7cc583b2f6db4e5fca8b1feffb
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 4 22:28:54 2011 +0200

    ouch

commit c7f78777ca6f0d20bf6fccb95f26ae92a667499a
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 4 22:21:11 2011 +0200

    execute everything again

commit 70306c72a153fd8828bf5c8bcaa2fee5b92ce508
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 4 22:11:13 2011 +0200

    comment out

commit 13205842c7231cf2ed5fc385394dfd712b97c34d
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 4 21:22:20 2011 +0200

    test

commit f65c523fc99cb78695890b4cc4f80385d80bf122
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 4 21:21:06 2011 +0200

    test

commit 1c978d4f5c4ddc5599055d0db7f49e6b1f75bccf
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 4 20:57:09 2011 +0200

    a lot of fixes

commit a590edf963c6dd57badbdb624afe283826048b39
Merge: 02a66d1 a4ec51a8
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed May 4 20:43:56 2011 +0200

    Merge branch '7.x-3.x' into 681468-display-extend

commit 02a66d1c5119bcfdc455835163b99788c7dd162d
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Sun May 1 09:00:42 2011 +0200

    initialize the display extender

commit 43926bce071192924b2de3a75554ea52b5b962c0
Merge: 42a8c55 74591cb8
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Sun May 1 08:15:31 2011 +0200

    Merge branch '7.x-3.x' into 681468-display-extend

commit 42a8c554936d66d7916c17cfeb0312084937db55
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Wed Apr 27 20:37:04 2011 +0200

    update

commit a095c54fce629f516005bb1e5060de9a53120121
Merge: 6a06ea3 21216dd1
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Mon Apr 25 19:38:06 2011 +0200

    Merge branch '7.x-3.x' into 681468-display-extend

commit 6a06ea35ab93af89dd7b64566173e846fdd947c7
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 22:55:57 2011 +0200

    add class to registry

commit 33f27a8e259b838b9c0091869327697a023efdba
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 22:50:06 2011 +0200

    remove unneeded row

commit a026dad1cd009364d048f2cf3a85ca8ce22e128c
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 22:43:32 2011 +0200

    add options

commit 6f7dc22839ea2859a6784a490758b9deeb3b1f59
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 22:27:03 2011 +0200

    add option definition

commit ba97ab0670103d50af4d2a31658b0bcf94a564f1
Merge: d36ffea 42047bef
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 22:21:10 2011 +0200

    Merge branch '7.x-3.x' into 681468-display-extend

commit d36ffeacca85483909e517c8d8de253faa51576b
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 22:20:34 2011 +0200

    merge in all functions

commit 5b374ad8bf5be7fc0311a5939b6c904e983c40fe
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 22:13:14 2011 +0200

    some functions explained

commit c5dbcd54bfcd1616df9ef7eb8255df7fbe7e1d50
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 22:07:29 2011 +0200

    add many possible functions to extenders

commit da761a45e6689ab54802f7c52e6035d4ec24e60f
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 21:53:35 2011 +0200

    add display extender plugin

commit a3269de37e2436887c2adbe3bb48c1bb44ff7344
Author: Daniel Wehner <daniel.wehner@erdfisch.de>
Date:   Fri Apr 22 21:53:22 2011 +0200

    default display extender