This patch adds =frame-selected notification to GDB/MI. This
notification is important, because it provides front-ends a way to find
out that user has asked for frame change via frontend console using CLI
command `up`, `down` or `frame N`.

I had to move `frame_info` structure description to `frame.h` so its
fields may be accessed from mi-main.c. I'm not sure if this is the
correct approach.

Nick Roberts once posted a patch which implements both =thread-selected
and =frame-selected via observers:
http://sourceware.org/ml/gdb-patches/2008-04/msg00377.html. Current
implementation of =thread-selected in mi-main.c uses a different
approach, comparing current thread ID before and after executing a
command in `mi_execute_command` function. My patch does the same for
frame level. I don't know how is it technically better. I think it would
be nice if somebody reviewed Nick's patch once again or at least
accepted the one proposed by me.