Allow assign/select operators to execute when armature tab is pinned.
For assign operators, pinned armature should be active in 3D view
because we cannot assign bones of one armature to the bone collection of
different armature. Added extra condition in poll function to control
the above case.
For select operators, use `ED_armature_context` to obtain pinned Armature.
These operators are now hidden in object modes.
Pull Request: https://projects.blender.org/blender/blender/pulls/137587
3aea40b8cd