Move OpenVR init function out of Overlaycontroller

This prevents the issue of OpenVR not being initialized in constructors
of other objects, and there's really no reason not to do it.

src/overlaycontroller.cpp

@@ -31,8 +31,7 @@ QSettings* OverlayController::_appSettings = nullptr;
 OverlayController::OverlayController( bool desktopMode,
                                       bool noSound,
                                       QQmlEngine& qmlEngine )
-    : QObject(), m_desktopMode( desktopMode ), m_noSound( noSound ),
-      m_openVrInit(), m_actions()
+    : QObject(), m_desktopMode( desktopMode ), m_noSound( noSound ), m_actions()
 {
     // Despite arguably being OpenVR init code, the call is still here because
     // the TabController uses this directly. Offering it through OpenVR_Init