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
• [background image] Make background image deletable/cut/copy/paste #1788
• [background image] Make image data participate in copy/paste #1793
• [background image] Add support for background image in the selection info panel #1803
• [background image] Allow a background image to be copied as image data (in addition to "normal" copy) #1796
• [background image] Add a context menu to "lock background image": this should be a global flag, not a per-image flag #1800
• [background image] Allow a PNG or JPEG image to be pasted onto a glyph in edit mode #1798
• [background image] Allow a PNG or JPEG file to be dropped onto a glyph in edit mode #1807
• [background image] Allow a PNG or JPEG image to be selected from the file system #1812
• [background image] [designspace] Convert JPEG to PNG with Pillow as part of putBackgroundImage() #1809
• [background image] Support colorization of the background image #1814
• [background image] Clean up unused background images from .ufo and .fontra #1813

Completing this issue also completes #1660.