x11-libs/gtk+: bump to 3.24.0 for a few new features

x11-libs/gtk+: bump to 3.24.0 for a few new features

Most notable new features for the gtk3 series with 3.24 over 3.22 are:
* The font chooser exposes more advanced OpenType features
* Emoji completion support via :emoji-name: style suggestions
* New GdkEventController subclasses for more fine-grained control
  over input events.
* Ability to choose color from the screen with Wayland. Requires
  running under newer gnome-shell than we have at the time of
  commit or a newer kwin, though.

3.24.x is now really expected to be the last gtk3 series, but bug fix
releases are expected to happen for quite some years to come. Next big
thing is gtk4, expected in March 2019.

Package-Manager: Portage-2.3.49, Repoman-2.3.10