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

#424176: Panel nodes: Remove unused layout screen, fix some notice errors, give context a description.

Some major restructuring, catching up to CTools. This is very much in process, there is a huge list of things that need to happen here still, but this may be checkpoint usable to the strong-stomached.

Initial D6 work -- this includes only panels.module stuff. Right now the drag & drop display editor works, layout, layout settings and render works. Special features such as cache and styles may or may not work.