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First draft of new resize user control

See #115.

This first draft was all about moving the resize UI code out of the
Image Resize dialog and into a standalone control and getting everything
working again.  This is mostly done.

Integration with the command bar (e.g. last-used settings) is not
finished yet, and the control does not yet theme properly, particularly
its font settings.  I'm working on it.
Add custom font support to new resize UC
Add dev warning to Resize dialog

I've got some good updates to sync, but the resize dialog will remain a
bit wonky until work on the new resize UC completes.  As I'm not doing
the work on a separate branch (shame on me, I know), I'm going to simply
add a warning to the dialog for now.