Add confirmation and removal to bindings menu

This makes the following improvements to the gamepad bindings menu:

- The menu now shows a hint that you can press a button while any of
  the bind options are selected (or that you can navigate away from
  those options)
- Instead of button presses immediately setting a binding, they now
  ask for confirmation: press the same button a second time to confirm
- You can now remove a binding, the same way you add it (this has the
  same type of confirmation)
- This menu used to be inconsistent with pretty much every other menu
  in the game by showing a permanent title and description for the menu
  itself ("Game Pad", "Change controller options.") rather than showing
  a title and description for the currently selected option.
  This inconsistency is now fixed.

desktop_version/src/Game.h

@@ -404,6 +404,10 @@ public:
     int creditposx, creditposy, creditposdelay;
     int oldcreditposx;
 
+    SDL_GameControllerButton gpmenu_lastbutton;
+    bool gpmenu_confirming;
+    bool gpmenu_showremove;
+
     bool silence_settings_error;