Cleanup, Gen. Popup impl. #20

* Renamed widget filenames (no more "nxs_ext_" prefixes
* Removed the "cursors" for the containers header, sidebar, footer and
pagelets
* Added a 2nd page settings button for posts; clicking on it opens the
layout popup that can be used to change header, sidebar, subheader,
subfooter and footer.
* Opening new edit sessions now supports a "sheet" parameter.
* Moved paintroller icon one above for widgets.
* Added a "fallback" scenario for options for widgets.