Add new use flag "spell" for enabling spell checking without full
KDE integration. KF5Sonnet is categorized as "Optional KF5 tier1
component" and can be used without full KDE, if find_package finds
it.
Using this new USE flag we disable automagic dependency on KF5Sonnet
if it is installed during build.
Changed REQUIRED_USE so that kde depends on spell, and spell depends
on some kind of gui.
Signed-off-by: Arthur Zamarin <arthurzam@gentoo.org>
Closes: https://github.com/gentoo/gentoo/pull/22531
Signed-off-by: Sam James <sam@gentoo.org>