Rigify uses a property group to contain options of its rigs, so
currently it is impossible to use Alt-Click or Copy To Selected
to change a setting for multiple rigs at the same time.
The main problem here is that there is no efficient way to find
which bone the property group belongs to. To maintain performance,
implement this by checking the active bone if it is known. Copy
Data Path and related features still don't work, as data path
calculation can't use context.