added code for overlay touch buttons (currently not used)

added code for overlay touch buttons (currently not used)

This adds support for overlay touch buttons to be used with the
Android port, to address the following problems:

* Especially when playing on smartphones, the display may be too small
  to use the most essential buttons, like the "yes" and "no" buttons
  of the door request dialog, or the "stop" and "pause" buttons on the
  game panel.

* Additionally, the most important game panel buttons (again, mainly
  the "stop" and "pause" buttons) may not be accessible at all because
  of being covered by virtual buttons.

* When configuring virtual buttons using recent Android versions,
  navigating to the next page is not possible anymore, because there
  is no "menu" button anymore. This was a hardware button opposite
  to the "back" button, so the "menu" and "back" buttons could be used
  to navigate to the next or previous page when configuring virtual
  buttons.

* When thinking about a potential iOS port of the game, there is not
  even a "back" button (like on Android devices), so it is even more
  difficult to stop playing a game (as the "stop" button has to be
  touched instead of just hitting the "back" button, which simply does
  not exist on iOS devices).

The next steps will be to use overlay touch buttons to solve some or
all of the above problems.