Early WIP for entity multi selection

[Cheaterman]

Need more eyes and hands on this, both for review and for testing :-) - it's still VERY buggy and somewhat incomplete ; documentation also needs to be adjusted, and some docstrings in the (admittedly very complex) code are sorely needed too.

[Cheaterman]

This is meant to address #50 BTW :-)

[hlorus]

This looks great as far as i've looked.

What exactly is very buggy, the core selection system itself or the adoption in the rest of the addon's operators?

Also how ar the entities in the hover list sorted? IMO defaulting to always showing a list of entities when selecting is bad as this would be the case for every line endpoint. Therefore it would be great if we could be sure that the first entity in the list is a reasonable choice and just pick that by default. Then use alt+lmb for the list.

As far as th "reasonable decision" goes, currently points are just always drawn before lines/circles etc which seems to work ok in most cases.

[hlorus]

This doesn't seem to work on my system. Usually there's just one entity highlighted or suddenly all entities are highlighted. Also i'm getting the following traceback when creating a simple rectangle and hovering an edgepoint:

KeyError: 14
Traceback (most recent call last):
File "/home/yoga/Dropbox/blend/addons/CAD_Sketcher/gizmos/preselection.py", line 67, in test_select
indices = rgba_to_indices(r, g, b, alpha)
File "/home/yoga/Dropbox/blend/addons/CAD_Sketcher/utilities/index.py", line 79, in rgba_to_indices
indices.append(_selection_to_index[index])