Make the source code of the menu interface more readable.
* normal/menu.c: Include grub/mm.h.
(TERM_WIDTH): New macro.
(TERM_HEIGHT): Likewise.
(TERM_INFO_HEIGHT): Likewise.
(TERM_MARGIN): Likewise.
(TERM_SCROLL_WIDTH): Likewise.
(TERM_TOP_BORDER_Y): Likewise.
(TERM_LEFT_BORDER_X): Likewise.
(TERM_BORDER_WIDTH): Likewise.
(TERM_MESSAGE_HEIGHT): Likewise.
(TERM_BORDER_HEIGHT): Likewise.
(TERM_NUM_ENTRIES): Likewise.
(TERM_FIRST_ENTRY_Y): Likewise.
(TERM_ENTRY_WIDTH): Likewise.
(TERM_CURSOR_X): Likewise.
(draw_border): Use macros instead of magic numbers.
(print_entry): Likewise.
(print_entries): Likewise.
(run_menu): Likewise. Also, handle the key 'e'.
(run_menu_entry): Ignore empty command lines.
(print_message): Added a new argument EDIT. If EDIT is true,
print a different message.
(init_page): Likewise.
(edit_menu_entry): New function. Not implemented yet.
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