Implement ShowSubtitles and HideSubtitles via keyboard and DBUS

Press the 'w' key above the 's' key to Show Subtitles
Press the 'x' key above the 's' key to Hide Subtitles

Also implements ShowSubtitles and HideSubtitles via DBUS
Adds ToggleSubtitles to DBUS

ie dbuscontrol.sh ToggleSubtitles
ie dbuscontrol.sh HideSubtitles
ie dbuscontrol.sh ShowSubtitles

Useful for machine-to-machine control of subtitles via DBUS where there is a requirement to explicitly set subtitles to a known state rather than toggle between two states.