Add variable tab

This should make it significantly easier to get an overview of the
variables, which currently exist, modify their settings, or potentially
remove multiple variables.

lib/advanced-scene-switcher.cpp

@@ -111,6 +111,7 @@ void AdvSceneSwitcher::LoadUI()
 	SetupSceneGroupTab();
 	SetupTriggerTab();
 	SetupMacroTab();
+	SetupVariableTab();
 
 	SetDeprecationWarnings();
 	SetTabOrder();