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

#460200: Allow regions and columns that have only 'fixed' width items. Disallow changing a 'fixed' item to 'fluid.'

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

#375686: Create panels_get_current_page_display() to replace the old panels_get_current_page() function.

Added allowed layouts to settings form for panel pages, nodes, minis.