Added a new concept to the menu, the ID.

Added a new concept to the menu, the ID.
Each item can have an ID that can be given, this way there won't be any more hardcoded values for the entries.
Anyway, I've made it to be compatible with the current behaviour, in case any ID is given.

I've already updated the mainmenu, optionsmenu, leveleditormenu, leveleditorsettingsmenu and worldmapmenu.
The definitions are enumerated in the menu.h file. Should we keep each enum splited or merge them all?

Feedback is appreciated :)

SVN-Revision: 674