Allow matching media players with more than one identifier

[ungive]

Identifying media players by arbitrary properties (#17 ) is more complicated than it needs to be.

For SMTC there is only the AppUserModelId. On Mac there is only the bundle identifier (or parent application bundle identifier). On Linux there is only the MPRIS service name and the "Identity" property. On the web there's the domain name.

Instead of allowing any property to be matched, it's sufficient to assign labels to specific properties and use them or a combination of them:

sources:
  lin_mpris:
    - tidal-hifi # "service" by default
    - service: tidal-hifi # same as previous line
    - service: chromium
      identity: tidal-hifi
    - identity: whatever

For Linux, "service" must always be present. "identity" should be matched against org.mpris.MediaPlayer2.DesktopEntry. Additionally, "desktop_entry" should also be added to match against org.mpris.MediaPlayer2.DesktopEntry, if the need ever arises. That could theoretically be matched on its own then too (e.g. with Strawberry the desktop entry contains org.strawberrymusicplayer.strawberry).

For completeness:

sources:
  win_smtc:
    - foo.exe
    - app_user_model_id: foo.exe
  mac_bundle:
    - app.foo
    - bundle_identifier: app.foo
  lin_mpris:
    - foo
    - service: foo
    - service: chromium
      identity: Foo
    - service: chromium
      desktop_entry: app.foo.desktop
  web_domain:
    - foo.com
    - domain: foo.com

Matching an empty string should be allowed with secondary properties ("identity" and "desktop_entry"). In that case the property must be actually empty.

[ungive]

In the case of MPRIS service should be mandatory, otherwise one would have to read properties of all services, which would be inefficient. Additionally every service should have some static component and not change continuously.