Blender Git Loki

Armature Make/Clear Parent: Grey out options that don't do anything

In armature edit mode, the Make/Clear Parent operators don't do anything
in various cases, but only one of these cases was previously indicated,
and it was indicated by hiding the option completely instead of graying
it out.

Clear Parent (Alt+P) problems fixed:
- "Clear Parent" option always showed up, even when none of the selected
bones had a parent.
- "Disconnect Bone" option always showed up, even when use_connected on
all selected bones was already false.

Make Parent (Ctrl+P) problems fixed:
- "Keep Offset" option didn't show up when all selected bones' parent
was already the active bone. This was correct, and this patch tries to
make all behaviours consistent with this.
- "Connected" option always showed up, even when all selected bones'
parent was already the active bone, and they all had use_connect set
to True.

With this patch all options show up all the time, but in cases where
they would do nothing, they will be grayed out.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D6100