ui/ncurses: Fix boot editor segfault on update

ui/ncurses: Fix boot editor segfault on update

The boot option editor screen segfaults on a system info update since it
loses track of which fields actually exist.

The boot editor screen's setup and drawing logic is a bit different from
other screens, so to fix this bug and preserve the maintainer's sanity,
bring the screen setup and redraw into line with other screens. This
includes a full teardown of the widgets on update, so save the content
of any textboxes on update so the user's changes are not lost.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>