Pulled panels page settings out and created a panels_common.inc so that panels_* can use some common features if they need them.

#149585: quasi-fix for devel.module interference. Of course, currently devel.module is broken and does not properly detect this so it interferes anyway, even though it claims not to.

#149343: When changing layouts, content would disappear if moved to a panel that already contained content.

#149375: New "add content" in the panels modal -- with categories and a bunch of new icons. Thanks *very* much yoroy!

#146169: Send the panels AJAX URL via a drupal setting to display_editor.js so that the URL will be appropriate to the Drupal installation.