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/SettingsTabController.cpp

@@ -8,7 +8,7 @@ namespace advsettings
 void SettingsTabController::initStage1()
 {
     m_autoStartEnabled = vr::VRApplications()->GetApplicationAutoLaunch(
-        OverlayController::applicationKey );
+        application_strings::applicationKey );
     auto settings = OverlayController::appSettings();
     settings->beginGroup( "applicationSettings" );
     auto value = settings->value( "forceRevivePage", m_forceRevivePage );
@@ -33,7 +33,7 @@ void SettingsTabController::eventLoopTick()
         if ( parent->isDashboardVisible() )
         {
             setAutoStartEnabled( vr::VRApplications()->GetApplicationAutoLaunch(
-                OverlayController::applicationKey ) );
+                application_strings::applicationKey ) );
         }
         settingsUpdateCounter = 0;
     }
@@ -54,7 +54,7 @@ void SettingsTabController::setAutoStartEnabled( bool value, bool notify )
     {
         m_autoStartEnabled = value;
         auto apperror = vr::VRApplications()->SetApplicationAutoLaunch(
-            OverlayController::applicationKey, m_autoStartEnabled );
+            application_strings::applicationKey, m_autoStartEnabled );
         if ( apperror != vr::VRApplicationError_None )
         {
             LOG( ERROR ) << "Could not set auto start: "