Canvas UC: fix erroneous resetting of layer when mouse leaves canvas but interaction has occurred
Selection class: provide "requestThumbnail" function

A thumbnail of the current selection shape is used by the new Undo
History browser.
Edit > Undo History: now implemented

Most software provides this as a floating toolbar, which is nice, but
that's going to require quite a bit of extra work in PD due to the
window manager, ugly interactions with per-state interface
synchronization, and the lack of an accordion panel UI framework.

To that end, this first implementation is just a modal dialog - not the
best solution in the world, but enough to thoroughly test the feature
and get various related bits implemented, so I'm content to leave the
"floating toolbox" change for some future date, after other UI
requirements and frameworks are in place.