tap_ui -> stat_tap_ui.

The old routine had stat_ in the name, as the expectation was that they
were for statistics taps; that's still the expectation, so have stat_ in
the data structure and routine names.

Change-Id: Ic98d011012b8641173d41fa0ec4f4e625614370a
Reviewed-on: https://code.wireshark.org/review/5303
Reviewed-by: Guy Harris <guy@alum.mit.edu>