allow FS mapping of geometry restricted windows

and copy isSpecialWindow() check as rulebook input
to setFullscreen()

Client::isFullScreenable() checks:
* fullscreen rule
* fullscreen_hack (-> for normal windows)
* geometry restrictions
* special window

Client::manage() for fullscreeining checks:
* fullscreen rule (with correct "initial" parameter)
* fullscreen_hack

-> this breaks the fullscreen rule for geometry restricted windows
and causes inconsistent behavior between client requests at runtime
(which do not test ::isFullScreenable()) and on mapping.

Otoh, the specialWindow() protection should apply generally - those
kind of windows should not be fullscreened since the user can not
exit this state via kwin for them - and there's hardly a good reason
for them to be fullscreen, esp. not to enter that state at runtime

REVIEW: 118442
CCBUG: 335617

Cherry-picked from kde-workspace
43229afee9fac4303e3d280ea63f96f034b3ffb5