[background image] Add a dialog for image properties

[ollimeier]

mostly color, or maybe just opacity/alpha

[justvanrossum]

To be discussed:

It occurred to me that it may be nice to have the bg image have its own section in the Selection Info panel. It would be similar to a component item: the transform block can be the same. Additionally an opacity slider.

This could be instead of a dialog.

Perhaps we should consider this for anchors and guidelines, too? And get rid of those dialogs?

[justvanrossum]

I went for the selection info panel solution, so this issue is superseded by #1803/#1804.