Background Images, part II

[justvanrossum]

As follow-ups to #1775, the following needs to be done (as separate tasks wherever possible):

• [background image] Make the background image selectable, editable, etc. #1782
  • selectable (+ "selected" visualization, I imagine a stroke around the image)
  • editable (movable, draggable, via EditBehavior?)
  • transformable ("bounds handles" resize/rotate)
  • deleteable
• add a dialog for image properties (mostly color, or maybe just opacity/alpha)
• add a context menu to "lock background image": this should be a global flag, not a per-image flag
• allow a .png or .jpg image to be dropped onto a glyph in edit mode
• allow a .png or .jpg image to be pasted onto a glyph in edit mode
• allow a .png or .jpg image to be selected, to be put onto a glyph in edit mode
• make background image data participate in copy/paste
• allow a background image to be copied as image data (in addition to "normal" copy)
• designspace backend: convert .jpg to .png with Pillow as part of putBackgroundImage()
• determine a scheme when to delete the underlying image data (tricky, as it doesn't participate in undo)

For the first item, I guess some of the code from #1707 can be used.

Completing this issue also completes #1660.