Add property helpers, refactor sdl2gui and guibase.
Also:
- Add int toInt(size_t) function because of size_t issues on x64. SHOULD investigate!
- Add enumutils for testing if (a) specific flag(s) were raised.
- Add callCounter mixin.
- Add new exception types SceneNotFoundException and InvalidSceneException.
- Refactor .gitignore.
- Refactor dub.json.