Convert images to use .qrc file, move images

All images have been moved to the /img/ folder in order to fully
separate qml from images. The .qrc file allows qml to use the images by
appending the qrc:/ prefix before the abstract path to the image.

Note that the .qrc file is also check at compile time, which further
enhances the confidence the lack of errors.

Some of the images are not used in the QML, but instead directly passed
through to the OpenVR API. These have not been added to the .qrc file.

src/overlaycontroller.cpp

@@ -332,9 +332,9 @@ void OverlayController::SetWidget( QQuickItem* quickItem,
             m_ulOverlayHandle,
             vr::VROverlayFlags_SendVRSmoothScrollEvents,
             true );
-        QString thumbIconPath
-            = QStandardPaths::locate( QStandardPaths::AppDataLocation,
-                                      QStringLiteral( "res/thumbicon.png" ) );
+        QString thumbIconPath = QStandardPaths::locate(
+            QStandardPaths::AppDataLocation,
+            QStringLiteral( "res/img/icons/thumbicon.png" ) );
         if ( QFile::exists( thumbIconPath ) )
         {
             vr::VROverlay()->SetOverlayFromFile(