Improve the renderer selection framework. This framework has no UI, but it utilizes exportables and new choices can be hand crafted.

#844062: Turned the normal drag and drop editor into a renderer plugin, made the IPE part of it. Also some fairly serious code reorganization.

variant creation wizard.

calling it 'default' was misleading since sometimes the legacy renderer
actually acts as the true 'default' (the renderer that's used when no
other one is specified).

Turn off ajax load for Selection Criteria tab in Page Manager, such that access plugins can properly load their own js files.

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

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.

Clean up caching of displays in page manager. Still need to do mini panels and panel nodes, I suppose.

Update CSS caching on panel page and panel context to use ctools_css_store instead of remembering the filename.