gclue-simple: Add portal support

When running inside a flatpak sandbox, or when GTK_USE_PORTAL
is set in the environment, talk to the location portal instead
of GeoClue directly.

In this situation, gclue_simple_get_client will return NULL.
3 jobs for portal-support in 17 minutes and 5 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Test
passed #943160
ubuntu-16.04

00:17:05

passed #943161
ubuntu-18.04

00:15:49

passed #943162
ubuntu-18.04-no-backend

00:09:04