Rename and move ui/stat_menu.h to epan/stat_groups.h.

The groups are, technically, independent of the notion of a menu, and,
if we have mechanisms by which taps that are not only GUI
toolkit-independent but independent of the *existence* of a GUI can be
registered, they might want to register themselves in a group just in
case they're running in a program that has a GUI.

Also, this might fix the Debian package build.

Change-Id: I29435681e79748fd4f2e0c5ac872cd11f831d172
Reviewed-on: https://code.wireshark.org/review/2830
Reviewed-by: Guy Harris <guy@alum.mit.edu>