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/ChaperoneTabController.cpp

@@ -540,8 +540,7 @@ void ChaperoneTabController::handleChaperoneWarnings( float distance )
         {
             if ( !vr::VROverlay()->IsDashboardVisible() )
             {
-                vr::VROverlay()->ShowDashboard(
-                    OverlayController::applicationKey );
+                vr::VROverlay()->ShowDashboard( application_strings::applicationKey );
             }
             m_chaperoneShowDashboardActive = true;
         }