MPRIS D-Bus interface does not conform to spec

Describe the bug

Some (or all) D-Bus org.mpris.MediaPlayer2 properties are not exposed as "variant" types.

To Reproduce

Execute this command to see the type of e.g. the org.mpris.MediaPlayer2.Identity property:

$ dbus-send --print-reply   --dest=org.mpris.MediaPlayer2.harmonoid   /org/mpris/MediaPlayer2   org.freedesktop.DBus.Properties.Get   string:"org.mpris.MediaPlayer2" string:"Identity"
method return time=1762894391.720889 sender=:1.26144 -> destination=:1.26211 serial=23 reply_serial=2
   string "Harmonoid"

Observe that the type is string.

Expected behavior

The output should be something like variant string "Harmonoid".

Screenshots

Here's the output for Spotify:

$ dbus-send --print-reply   --dest=org.mpris.MediaPlayer2.spotify   /org/mpris/MediaPlayer2   org.freedesktop.DBus.Properties.Get   string:"org.mpris.MediaPlayer2" string:"Identity"
method return time=1763029958.984578 sender=:1.347 -> destination=:1.1610 serial=7433 reply_serial=2
   variant       string "Spotify"

All other media players I know of also report variants.

Related issues:

• https://github.com/Kistler-Group/sdbus-cpp/issues/518
• https://github.com/ungive/discord-music-presence/issues/444

Platform (please complete the following information)

• OS: Linux
• Version [e.g. Windows 11 19041, Fedora 33, Android 12 MIUI 13]

App version

Please confirm that you're on the latest version available.

• Version: v0.3.10