Add media key bindings, update docs

All bindings are disabled by default. This is to prevent confusing
behaviour from users who do not know about the new bindings. Forcing
users to manually set up their bindings will also enable them to play
around with the SteamVR bindings interface, which allows for much more
versatile binding options that our current offerings.

All actions are curently in main action set. If the selection of
bindings was greatly expanded it could be beneficial to encapsulate the
options more. The current code is more intended as a proof of concept
until our full binding needs have been uncovered.

The processInputBindings function should probably also be split into
several functions of related bindings, as the amount of bindings grows.

src/overlaycontroller.cpp

@@ -471,6 +471,33 @@ QPoint OverlayController::getMousePositionForEvent( vr::VREvent_Mouse_t mouse )
     return QPoint( mouse.x, y );
 }
 
+/*!
+Checks if an action has been activated and dispatches the related action if it
+has been.
+
+This function should probably be split into several functions that are specific
+to a binding type as the amount of actions grows.
+*/
+void OverlayController::processInputBindings()
+{
+    if ( m_actions.nextSong() )
+    {
+        m_utilitiesTabController.sendMediaNextSong();
+    }
+    if ( m_actions.previousSong() )
+    {
+        m_utilitiesTabController.sendMediaPreviousSong();
+    }
+    if ( m_actions.pausePlaySong() )
+    {
+        m_utilitiesTabController.sendMediaPausePlay();
+    }
+    if ( m_actions.stopSong() )
+    {
+        m_utilitiesTabController.sendMediaStopSong();
+    }
+}
+
 void OverlayController::OnTimeoutPumpEvents()
 {
     if ( !vr::VRSystem() )
@@ -478,10 +505,7 @@ void OverlayController::OnTimeoutPumpEvents()
 
     m_actions.UpdateStates();
 
-    if ( m_actions.nextSong() )
-    {
-        LOG( INFO ) << "NEXT SONG SET!";
-    }
+    processInputBindings();
 
     vr::VREvent_t vrEvent;
     bool chaperoneDataAlreadyUpdated = false;