build-sys: bump gtk requirement 3.12 -> 3.22

Fedora 28: 3.22.30
Fedora 27: 3.22.26
Fedora 26: 3.22.21
RHEL 7.6: 3.22.30
RHEL 7.5: 3.22.26
RHEL 7.4: 3.22.10
Debian 10: 3.22.30
Debian  9: 3.22.11
Ubuntu 18.04: 3.22.30
Ubuntu 17.10: 3.22.24

Besides cleanup the code, gtk is under heavy development to better
work under Wayland and reducing the code path based on library version
should be helpful as well.

This patch also removes the check for gdk_event_get_scancode as it is
included in 3.22.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Signed-off-by: Frediano Ziglio <fziglio@redhat.com>