Overhaul of custom rigs management

[joepal1976]

This feature request is a merge of a bunch of issues related to custom rigs:

• Refitting fails on a custom rig #305 (refitting fails on custom rig)
• Refactor rig creation tools #114 (refactor rig creation tools)
• Should be possible to add and use a custom rigify rig #42 (custom rigify rigs)
• Should be possible to use custom-made or third part rigs #39 (custom standard rigs)

These issues conceptually all relate to the same thing: Custom rigs should be possible to create and use in the same manner as other assets. Currently, only the provided rigs are recognized by the system.

Planned activities:

• Move everything related to rig creation to panel(s) under "create assets" (there is already a "MakeRig" there with some functionality)
• Read and recognize rigs saved in the user data
• Add "save to library" functionality for save rig and save weight

There are probably other related changes needed.