Wait with screen unload until next screen is loaded and ready to be shown (#74)

* Removed screen flicker when transitioning between screens

* Moved advanced example to subfolder. Added basic example.

* Remove flicker on back navigation too

* Fix issue with no_stack screens added at any time

test/data/screen_preload.gui_script

@@ -1,5 +1,5 @@
 function init(self)
 	local monarch = require "monarch.monarch"
 	local data = monarch.data(hash("screen_preload"))
-	data.count = data.count + 1
+	if data then data.count = data.count + 1 end
 end