Fix default mpris icon size

[abmantis]

The widget was was using the .ui size instead of the user config size in the following situations:

• there was no media player app
• the media player app icon was used
• the audio-x-generic-symbolic icon was used

This fix sets the default size for the album art icon according to the user's config.

set_from_gicon and set_from_icon_name receive a Gtk.IconSize enum value, not an int, so we need to call set_pixel_size to set the proper size.

[ErikReider]

Could you also update the branch off of main?