app, build: depend on GLib 2.70
@Jehan, g_thread_pool_new_full
(used since commit 52d17439 in xcf-save) is available since GLib 2.70, but in [autotools|meson] build files we still list dependency on 2.68.
I'm not sure whether it was just an oversight or we can't bump GLib version some reason. That's why I don't push directly to master, but wire through MR.