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.h

@@ -90,9 +90,10 @@ private:
     // the class.
     openvr_init::OpenVR_Init m_openVrInit;
 
+    input::SteamIVRInput m_actions;
+
 public: // I know it's an ugly hack to make them public to enable external
         // access, but I am too lazy to implement getters.
-    input::SteamIVRInput m_actions;
     SteamVRTabController m_steamVRTabController;
     ChaperoneTabController m_chaperoneTabController;
     MoveCenterTabController m_moveCenterTabController;
@@ -106,6 +107,7 @@ public: // I know it's an ugly hack to make them public to enable external
 
 private:
     QPoint getMousePositionForEvent( vr::VREvent_Mouse_t mouse );
+    void processInputBindings();
 
 public:
     OverlayController( bool desktopMode, bool noSound, QQmlEngine& qmlEngine );