Fix enormous slowdown when changing from selecting an item to a single bone.

A similar issue had before as #25, but this one had a different root cause. In particular, the reason is that with all the combined bones and stuff, opening a single bone takes a lot longer than it used to. That made another issue very visible: Setting a single bone as the only selection caused first all bones to be selected, then all of them removed from the selection, then one added back in. That was kind of stupid. Now the selection is simply cleared, and then the single bone added.

The long time for finding children still needs to be addressed, though.
Cache parent, since that can't and won't change.

Another attempt to speed things up when selecting a lot of bones at once. Not yet the desired outcome.
Find the parent bone controller faster.

This solves the slowdown issue. The problem was that loading the parent controller asked every single other bone controller for its children, which caused a lot of work behind the scenes. This is no longer the case; now it only asks all bone controllers for their bone, which is incredibly faster.