[DolphinWX] Prevent hotkeys from being assigned to more than one button/function. Prior to this commit it was possible to assign the same keycode to more than one button. ie. Say I assigned Open with the hotkey Ctrl+O; well, it was possible to also add it to another function as well, which leads to hotkey clashing. Now, say I assign Open with Ctrl+O, but then assign that same hotkey to Refresh List; it will unbind the hotkey from Open and then assign it to refresh list.