Issues #10, #46 - Added history.dat plugin and related improvements.

- fe.get_input_state() can now test if any control mapped to a
  particular frontend action is pressed.  For example
  fe.get_input_state( "select" ) will return true if any button mapped to
  game selection is pressed.
- transition and tick callback functions inside squirrel tables or class
  instances can now be registered.
- Improvements for multiline text display in the frontend
- Added ability to run squirrel script functions from the config menus for
  layouts, plugins and the screensaver.
- Added a plugin for history.dat viewing.
- Added a "submenu" module to simplify creating submenus in
  layouts/plugins.
- Removed some deprecated script functions.