Add a "register_dfilter_stat()", to register stats that take a display

Add a "register_dfilter_stat()", to register stats that take a display
filter as an argument on the command line and have a dialog box to enter
the display filter through the GUI.  Use it for all stats using
"gtk_tap_dfilter_dlg_cb()".

Add a top-level "stat_menu.h" file to declare "REGISTER_STAT_GROUP_E"
for the benefit of the declaration of "register_dfilter_stat()" in the
top-level "tap_dfilter_dlg.h".  Rename the "stat_menu.h" in the gtk
directory to "gtk_stat_menu.h", so as not to have two headers with the
same name.

Get rid of headers not declaring any functions not being used in the
module.

svn path=/trunk/; revision=15493