Blender Git Loki

Add Extras Dropdown Menu to Constraints

Add Apply Constraint, Duplicate Constraint, and Copy To Selected
operators, and include them in a menu similar to the menu for modifiers.
The shortcuts in the extras menu are also matched to modifiers.

All the here added operators are intended to work exactly like the
analogous ones for modifiers. That means the apply operator should apply
a constraint as if it was first in the list, just like modifiers do. I
have added the same warning message as for modifiers when that happens.

The decision to use this approach of appling the constraint as if it was
first, was made for consistency with modifiers. People are already used
to how it works there. Is also provides more intricate control over the
applied transforms, then just applying all constraints up to that one.
Apply all constraints is already kinda implemented in Bake Animation.

Reviewed By: HooglyBoogly, sybren, #user_interface

Differential Revision: https://developer.blender.org/D10914