notify: Use GBytesIcon

Previously we sent a GFileIcon for notifications, but the
flatpak portal does not support GFileIcon (https://github.com/flatpak/xdg-desktop-portal/issues/317).
Because of this notifications from the flatpak were
being dropped.

Now we create a GBytesIcon by loading a GFile and reading
the bytes, and set that as the icon for notifications.

Closes GNOME/fractal#490
4 jobs for wip/christopherdavis/fix-notifications in 14 minutes and 40 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #296912
check

00:00:23

 
  Test
passed #296913
flatpak

00:13:35

 
  Review
passed #296914
review

00:00:40

failed #296915
allowed to fail manual
stop_review

 
Name Stage Failure
failed
stop_review Review There has been a missing dependency failure
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