Expose actions through D-BUS for external software to interact with Argos

[orontee]

Motivation

User may want to:

• Toggle dark theme depending on sunrise, sunset time
• Toggle playback at given time of day
• Refresh library at given time of day

Todo

• [x] Most "actions" should be implemented through Gio.Action and as a consequence be automatically exposed through D-Bus interface.

Many are missing with current implementation:

↳ busctl --user --json=short call io.github.orontee.Argos /io/github/orontee/Argos org.gtk.Actions DescribeAll | jq ".data[0] | keys"
[
  "add-stream",
  "add-to-tracklist",
  "browse-directory",
  "collect-album-information",
  "complete-album-description",
  "complete-playlist-description",
  "delete-playlist",
  "enable-dark-theme",
  "fetch-album-images",
  "new-playlist",
  "play",
  "play-next-track",
  "play-prev-track",
  "play-random-tracks",
  "play-tracks",
  "quit",
  "remove-from-tracklist",
  "save-playlist",
  "seek",
  "set-consume",
  "set-random",
  "set-repeat",
  "set-single",
  "set-volume",
  "show-about-dialog",
  "show-preferences",
  "toggle-playback-state",
  "update-library"
]

• [x] Add a section to README.rst file about "automation".