Port build system to meson
This MR merges the 'meson' branch, which ports zenity3's build system from autotools to meson.
There was a translation update that happened in the meanwhile, which has been cherry-picked.
The GTK minimum version was also bumped to 3.16. See: !9 (closed) (this MR will be reviewed in more detail once this MR is committed).
Lastly, a compiler warning is suppressed using the G_GNUC_BEGIN_IGNORE_DEPRECATIONS and G_GNUC_END_IGNORE_DEPRECATIONS macros, which we get "for free" anyway, since they were introduced in glib 2.32, and the minimum glib requirement for gtk 3.16 is 2.43.4.