grilo: Queue loading

Put all GriloMedia requests into a global queue and process them one by
one. This allows to avoid UI stalling while loading metadata for many
games at once at a cost of it taking longer time.
6 jobs for wip/exalm/cache2 in 35 minutes and 4 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed #119806
flatpak:app

00:25:52

 
  Test
passed #119807
flatpak:tests

00:05:29

 
  Review
passed #119808
review

00:00:21

manual #119809
allowed to fail manual
stop_review
 
  Cross Distro
passed #119810
fedora:29

00:03:20

passed #119811
ubuntu:devel

00:02:21