Flesh out layer menu

Not completely, just yet - Layer transforms (resize, rotate, etc) will
be done later.  For now, I mostly wanted a framework so I could start
plugging in various Layer menu actions.
Add layer menu placeholder code

All the layer menus are now ready to have processor calls plugged in.
Get layer menu enable/disable state worked out

It's fairly complicated, as things like layer position and count affect
what options are available (e.g. no Merge options for single-layer
images).  Hopefully it's all working as it should...
Mirror all layer box functionality in the Layer menu

Also, layer stack order changes now triggers Undo/Redo creation
Improve undo/redo performance

There's a ton of work to do on undo/redo performance (right now, a full
copy of the entire layer stack is saved to file for every action), but
it's quite unpleasant and complicated work.

In the meantime, this commit will help a little bit.  When working with
undo data, PD now:

- skips compression
- skips checksum calculation (when saving) and reading (when loading)
Temporarily disable undo/redo when raising/lowering layers

There are pros and cons to including non-destructive actions like layer
order changes in the undo/redo chain.

At present, undo file creation remains slow on very large images, so I'm
disabling it for basic "raise or lower layer" actions.
Add placeholder message boxes for incomplete Layer menu items