build: bump GTK requirement

https://bugzilla.gnome.org/show_bug.cgi?id=731110
connect-server-dialog: Use the new support for RTL icons in GtkIconTheme

https://bugzilla.gnome.org/show_bug.cgi?id=731110
location-entry: Use the new support for RTL icons in GtkIconTheme

https://bugzilla.gnome.org/show_bug.cgi?id=731110
toolbar: Use the new support for RTL icons in GtkIconTheme

https://bugzilla.gnome.org/show_bug.cgi?id=731110