Before, 3 Rigify panels:
{F12837079}

After, single Rigify panel with sub-panels:
{F12823002}

Originally mentioned in T88711, where it didn't spark much discussion, hopefully meaning it's non-controversial.

This patch proposes moving the many panels of Rigify into sub-panels under a single Rigify main panel, in order to reduce clutter in the Properties Editor's Armature tab. This also makes it easier to add more Rigify features in the future, since new panels won't make the overcrowding of this tab even worse.

It also unifies some of the available functionalities between edit and pose mode, where limiting a workflow to only pose mode didn't seem necessary.

Reviewed By: angavrilov

Differential Revision: https://developer.blender.org/D13914