Allow introspection python commands to be run during call-graph breaks

[hardenedapple]

It seems reasonably easy to call gdb python functions in the EntryBreak and ReturnBreak stop() methods.

More command options could be added to the call-graph command that define breaks with extra data printed on each startup.

I'm not sure the benefits outweigh the extra complexity (just in terms of the end user experience), so I'm not going to do this as yet.

I'm storing the thought here though...