Worked on GameMenu and other stuff

Changed the ResourceManager to return constant types in several
functions and changed other code accordingly.
Worked on GameMenu's GameSlot class.
Worked on the GameMenu itself, making it a bit more dynamic.
Dumped all the MenuControl inclusion into a single header and changed
around in code accordingly.
Made a StaticBox MenuControl and made it replace the ordinary
RectangleShape all over the code.
Created a SelectedGameMenu class.