Media player (using DBUS) widget and commands

[DemonTPx]

This adds a media player widget and commands using dbus.

It allows control over all players which are following the dbus MPRIS MediaPlayer2 specification, however if multiple players are started, the player you want to control needs to be selected (using the select+ or select- command).

The widget can be used to view the album art (tested with Spotify and Rhythmbox), the name of the selected player and conditional icons depending on the current playback status (eg: a stop button when playing and a play button when paused.)

[eduardopicado]

This is a great PR, I got it working with Tidal-hifi on Ubuntu!