Blender Git Loki

Weight Paint: add a pie menu for locking and unlocking vertex groups.

Provide different options for locking and unlocking vertex groups
using bone selection, accessible via a pie menu triggered via the
'K' hotkey. To implement a variety of operations, extend the old
operator with a new option to mask it by bone selection. If the
X Mirror option is enabled, selection is automatically mirrored.

This follows D6533 as the next step in improving accessibility of
vertex group locking during weight painting.

Differential Revision: https://developer.blender.org/D6618