Move application strings into separate namespace

Them being in the overlay controller required them to be static, which
forces the compiler to give them a memory location at compile time. They
also don't really fit into the overlaycontroller.

src/tabcontrollers/AudioTabController.cpp

@@ -64,7 +64,7 @@ void AudioTabController::initStage2( OverlayController* var_parent,
     this->widget = var_widget;
 
     std::string notifKey
-        = std::string( OverlayController::applicationKey ) + ".pptnotification";
+        = std::string( application_strings::applicationKey ) + ".pptnotification";
 
     vr::VROverlayError overlayError = vr::VROverlay()->CreateOverlay(
         notifKey.c_str(), notifKey.c_str(), &m_ulNotificationOverlayHandle );