cache artUrl when given same path

Do not use the default GMainContext

Using the default GMainContext may cause deadlocks when mpv-mpris is
loaded in an mpv frontend that use GLib.

Discard the playerctl stderr when listing players

This fixes a test failure when using glib 2.73.3.

playerctl with glib 2.73.3 emits a deprecation warning and that is
merged into the player list and the comparison with 'mpv' then fails.

When mpv is not present, the reason for that still needs to be printed,
so re-run the command when it failed or did not detect mpv is the player.

Remove fullscreen print statement

update GitHub actions versions

Ensure that the D-Bus directory for tests has suitable permissions

The D-Bus directory must not be writable by other users:

  dbus: Unable to set up transient service directory: XDG_RUNTIME_DIR "." can be written by others (mode 040775)

Fixes test failure when building with less restrictive umask 0002.

Detected-by: Reproducible Builds <https://reproducible-builds.org/>
See-also: https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/mpv-mpris.html

Set xesam:artist to channel name for YouTube videos

Fixes: #50

Fail build if no artefact found

youtube thumbnails as mpris:artUrl

Closes #34

Check if string is valid utf8 before passing to glib (fixes #19)