Improve auto-creating constraints

[hlorus]

Tools that automatically add constraints should check if the system still solves. If there's an error the constraints shouldn't be added.

Additionally it would be great to have the option to turn off automatic constraining with a global toggle and ideally also a modifier key.

[bonjorno7]

Do you have a list of tools which automatically add constraints?
So far it seems all active tools (toolbar on the left) can do it under the right circumstances, except Offset Entities.

[hlorus]

Probably these:

add_point_3d
add_point_2d
add_line_3d
add_line_2d
add_arc
add_rectangle

bevel
trim

[hlorus]

Maybe we could use a context manager for this, then adding constraints would look something like this:

with safe_constraining() as constraints:
constraints.add_coincident()
....

[bonjorno7]

Good idea.

[bonjorno7]

I've made a start here: #425