[background image] Front-End updates (selectable, editable and transformable)

[ollimeier]

This fixes #1782

[ollimeier]

@justvanrossum backgroundImageSelectionAtPoint currently returns a set, to make it equal to the other selection functions.
But for now, only one background image can be selected, so it could be a boolean, right?
But this would be inconsistent with the other selection functions. What do you think?
Do we limit the background image to one image only or do we eventually allow multiple images in future?

[justvanrossum]

Please keep it a set, both for consistency with the other methods, as well as for possible future expansion to multiple images.

[justvanrossum]

Will this PR implement all of #1782, or just part of it?

[ollimeier]

Will this PR implement all of #1782, or just part of it?

What do you prefer? Both is still possible: as separate PRs or merge them into one and then have all of #1782 in one PR.

[justvanrossum]

Please use a better title than "Make the background image".

[justvanrossum]

And use [background image] as a prefix for all PR and issue titles related to the bg image tasks.

[justvanrossum]

Since it does not make sense to merge a "selectable" PR separarate from "editable" and "transformable", these three should for sure be one PR. Depending on the complecity "deletable" may be better as a followup.

[justvanrossum]

So, when I say "separate PRs", I mean "sequential PRs", not "nested PRs", ie. please please please no PRs on PRs, that just brings utter chaos.

[ollimeier]

Please use a better title than "Make the background image".

I changed the title to: [background image] Front-End updates (selectable, editable and transformable)