Massive cleanup of the main menu

* Name main menu entries with an enum. No more magic constants for
  them.
* Merge the entry indexes with the MAIN_MENU_CASE_ constants.
* Fewer hardcoded coordinates in the code that draws the main menu.
  Now rows are drawn relative to the previous row.
* Get rid of a global variable that was used in parallel with the 'c'
  variable, which keeps track of the last selected entry in the main
  menu.