[RFA 0/3] add a few Python events

[Tom Tromey]

This started as a series to fix PR python/15622 -- adding a few new
events to the Python layer.  In particular it adds new_inferior,
inferior_deleted, and new_thread events.

After that I noticed that object ownership in some spots could be
clearer; and that adding a new event or new event type to Python was
too hard, so I added patches to fix these things.

Regression tested on the buildbot.

Tom