Improve reliability of clipboard URL parsing
Add menus for Layer Size and Orientation

My goal is to provide non-destructive on-canvas resize and (eventually)
orientation tools for Layers.  In addition to those, there will of
course be menu-based items for resizing and changing layer orientation -
that's what this commit provides.

This commit also includes an updated Image menu, with new options for
automatically fitting the canvas to layers.  (I use said option all the
time in GIMP!)  Content-aware resizing has also been moved to the Layers
menu.  Due to the nasty complexities involved, it will need to be
handled on a single-layer-at-a-time basis.
Add MsgBox warnings to unfinished menu entries
Get Content-Aware Resize working on layers

This is a rather exciting fix, as it's my first chance to test out PD's
"resize" user control against an individual layer tool, rather than a
full-image tool.  (I designed the user control with Layers in mind, but
haven't been able to test it until now.)  AFAICT, it works just fine,
which will greatly simplify my other Layer size dialogs.  Phew.