Option to disable media keys support

**Is your feature request related to a usage problem (not a bug)? Please describe.**
Ubuntu often fails to get media keys to control the right application when there is more than one capable of playing audio open at the same time. Since I always mean for the media keys to control Spotify, I turn off media key support to other applications, like Firefox. I can't do this to Freetube.

**Describe the solution you'd like to see implemented**
I would like to be able to turn off media key support in Freetube, similarly to how it is possible to do this in Firefox.

**Additional context**
I only mean for the media keys to control Spotify, and having Freetube and Spotify open messes that behavior up. For example, I often would like to pause the music I am listening to so that I can watch a video on Freetube... Instead video on Fretube plays and I have to go to Spotify, pause manually then go back to Freetube. Having an option to turn off this feature in Freetube would solve my problem.