Introduce the Panels stylizer module which builds on the CTools stylizer base to create user definable, recolorable styles.

#683162: Convert all plugins to use new $plugin = array() format. This is less brittle than the specially named hook.

#612704 by c960657: panels_get_pane_content() inappropriately created a blank context when no contexts were available.

#607420: Visibility rules did not appear in the list when added to a pane (though they still applied).

#606980 by Deciphered and sethfreach: Typo caused panel-pane class to disappear if another class was assigned.

Add a new field to allow setting a panel title from one of its panes.

#568218: Links to clone, export, delete/revert, enable/disable variants from the main summary page were incorrect and led to an operation trail not found message.