#314495: Tab to change layout of panel node missing; also fix menu items so they only show up for panel nodes.

Updated context callback to comply with changes stemming from the D6 menu system. (Can handle both a uid and a fully-loaded user object, now)

Updated panels_panels_include_directory() docblock to correct errors and provide a better explanation.

#311192: Significant refactoring of the menu system; arguments at various non-override paths should now be operational, along with some other intelligent additions to the system.

Unified context id generation into a single callback, panels_context_id(). The output pattern is identical to before, just a little less code overhead.

Also updated some comments and docblocks.

Fixed (boneheaded) bug that was preventing more than one overriding panel page from storing the backup router data properly.

This fix will _hopefully_ also take care of lots of unhelpful watchdog 'Specialized Panels router rebuild failed...' errors.

Minor docblock tweaks. Also added some new constants that may or may not make it into the release, depending on some upcoming work.