Stepped rotation + node tangents snapping

[pgilfernandez]

Related with #250 and #628 (comment)

While rotating you can use shift modifier for 15º snapping or ctrl/cmd for 1º snapping.
It's indeed compatible with numerical input (it doesn't block it), rotating with R and gizmo.

Note that it is a stepped rotation by 15 or 1 degrees starting with the original rotation value the object had, it is not a snapping rotation where the rotation snaps to "previously fixed canvas rotations" (0º, 15º, 30º, ...). BTW, it's the way Inkscape and other software does it.

Further implementation would be using variables instead, so the rotation steps are configurable via Preferences and make the modifier also configurable (this is more related with shortcuts configuration...), but I don't know if that part should wait for v1.1 as it needs to change that panel... let me know what do you think and we find the right place.

I'm going to investigate if this feature can be as easily be implemented for node tangents too...

[pgilfernandez]

Well, I was able to add snapping to node tangents as well for both normal path nodes and keyframe bezier tangent nodes at the timeline Graph view mode... pretty cool and handy 😄

In the way I found a regression that I also fixed with #651