Adds omxplayer dbus support for togglesubtitles, hidesubtitles, showsubtitles

Update omxplayer.cpp

Add support for ShowSubtitles and HideSubtitles.

Update KeyConfig.cpp

'w' key (above the 's' key that Toggles Subtitles) turns subtitle display ON ie ShowSubtitles
'x' key (below the 's' key) turns subtitle display OFF ie HideSubtitles

Update OMXControl.cpp

Implement Show and Hide Subtitles

Update dbuscontrol.sh

Adds omxplayer dbus support for togglesubtitles, hidesubtitles, showsubtitles

Update KeyConfig.h

Adds support for HideSubtitles and ShowSubtitles.

Update README.md

Show subtitles with the 'w' key (ie above the 's' key the toggles subtitles)
Hide subtitles with the 'x' key (ie below the 's' key the toggles subtitles)

Update KeyConfig.h

Adds support for HideSubtitles and ShowSubtitles