gtk: disable GTK Clipboard with a new meson option

gtk: disable GTK Clipboard with a new meson option

The GTK Clipboard implementation may cause guest hangs.

Therefore implement new configure switch: --enable-gtk-clipboard,

as a meson option disabled by default, which warns in the help
text about the experimental nature of the feature.
Regenerate the meson build options to include it.

The initialization of the clipboard is gtk.c, as well as the
compilation of gtk-clipboard.c are now conditional on this new
option to be set.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1150
Signed-off-by: Claudio Fontana <[email protected]>
Acked-by: Gerd Hoffmann <[email protected]>
Reviewed-by: Jim Fehlig <[email protected]>
Message-Id: <20221121135538[email protected]>
Signed-off-by: Gerd Hoffmann <[email protected]>