Add a python event registry for selected frame changes

[Maxime Coste]

Hello,

Here is a patch serie which permits python scripts to watch for selected
frame changes, ignoring changes which are triggered by implementation detail
(like executing a call command).

I know the real solution to select_frame would be for most function to take
a frame rather than relying on the current selected frame, however I am not
familiar enough with gdb internals to fix that.

Regards,

Maxime Coste.