Make option to show turns on the main screen.

"Liberal" version that shows the turncount unguarded by
nomenukey and ISMOBILE.  Hopefully this means that it'll
display properly on mobile, outside the map region's "DRAG" area.
But this display mode is only used by the mobile builds which I
don't have toolchains for, so can't test it.  :(

config.cpp

@@ -1173,6 +1173,8 @@ EX void initConfig() {
 
   param_i(min_cells_drawn, "min_cells_drawn");
 
+  param_b(show_turns, "show_turns", false)
+  -> editable("show turn count", 'T');
   param_i(menu_darkening, "menu_darkening", 2)
   -> editable(0, 8, 1, "menu map darkening", "A larger number means darker game map in the background. Set to 8 to disable the background.", 'd')
   -> set_sets([] { dialog::bound_low(0); dialog::bound_up(8); dialog::get_di().dialogflags |= sm::DARKEN; });
@@ -2443,6 +2445,7 @@ EX void configureInterface() {
       };
     });
 
+  add_edit(show_turns);
   add_edit(menu_darkening);
   add_edit(centered_menus);
   add_edit(startanims::enabled);