patch 9.1.1743: Haiku: no full-screen support

patch 9.1.1743: Haiku: no full-screen support

Problem:  Haiku: no full-screen support
Solution: Add support for toggling full-screen using the keyboard
          (rymdbar)

Makes toggling using keyboard possible. This change does not add any
`:fullscreen` command (Which currently only macVim has).

See https://www.haiku-os.org/docs/userguide/en/keyboard-shortcuts.html
for motivation on key combination used, as well as terminology choice.
With vim being inconsistent (`:help intro` suggests <A> and <M>, while
<Alt> is used at a dozen other places) following Haiku nomenclature
seems most appropriate.

closes: #18235

Signed-off-by: rymdbar <rymdbar@x20.se>
Signed-off-by: Christian Brabandt <cb@256bit.org>