[FREELDR] Some enhancements for the UI code. (#1763)

[FREELDR] Some enhancements for the UI code. (#1763)

- EditBox: Display the initial contents of the text buffer.
  This allows modifying already existing text in the passed buffer.

- Menu:
  * Make both MenuHeader and MenuFooter optional (but the latter is
    more "optional" than the former...).

  * Allow passing a user-provided "Context" structure to the key-press
    filter callback, and pass also the index of the menu item that has
    been selected.

- Minor formatting fixes.