Interactive resizing of selection by selection bounds

[ollimeier]

Fixes #1368 This also prepares for #603

[ollimeier]

Further note from Just: We will separate:

• computation of the selection bounds
• using the selection bounds

Computation stores the current selection bounds in a scene model attribute. The rest of the code will use that attribute, unless it’s undefined.

[justvanrossum]

Maybe add those notes to #1492 instead.