Add mpris slot

[pbui]

Pull Request Template

Description

This allows Epiphany to report what media is playing (ie. YouTube video) via MPRIS and allows for third-party control of the media playback as shown below:

As explained in the comment, snapd does not allow for MPRIS names that contains dots due to this regex:

https://github.com/canonical/snapd/blob/master/interfaces/builtin/mpris.go#L215

However, WebKit creates the MPRIS DBUS name based on the gtk_application_id, which in this case is org.gnome.Epiphany. This is an issue due to the snapd restriction above.

Rather than patching Epiphany to use a shorten version of the gtk_application_id, it is easier to just tell snapd to allow for org as the MPRIS name. This leads to the following AppArmor rule:

# https://specifications.freedesktop.org/mpris-spec/latest/
# allow binding to the well-known DBus mpris interface based on the snap's name
dbus (bind)
    bus=session
    name="org.mpris.MediaPlayer2.org{,.*}",

Which allows Epiphany to bind to the MPRIS DBUS session.

Type of change

Please check only the options that are relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

I played a YouTube video and then looked in GNOME to see if the video has playback controls in the notification center as shown in the screenshot. I was able to pause and play the video from the notification controls.

Test Configuration:

Ubuntu 24.04.2

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have checked my code and corrected any misspellings