Support for chrome's Global Media Control (MediaSession)

[dellagustin]

as shown on the image below, chrome introduced a central user interface for media control (finally! I wanted something like that for a long time...)

We probably need to consume some API (I need to check the documentation) to provide the metadata.

I already identified some bugs related to it:

• in our player, the UI does not properly react when the control is changed with that button.
  • This does not seem to be happening anymore
• when I click the podstation entry on the media controller, it does not navigate to podStation (this was actually not expected to work in the first place, as podStation's audio element is actually on the background page, since it has to remain playing when the tab is closed). - created a issue just for this - Clicking on the Global Media Control entry does not navigate to a podStation window #55
• using the stop function on the player does not remove the media session from the global media control - solved with b7b03ab

I plan to create issues for these in separate, for the meantime, I will also label this issue as bug.

some news about it: https://techcrunch.com/2020/01/16/chrome-gets-global-media-controls/

More refs:
https://w3c.github.io/web-roadmaps/media/control.html
https://w3c.github.io/mediasession
https://developer.mozilla.org/en-US/docs/Web/API/Media_Session_API
https://github.com/w3c/mediasession/blob/master/explainer.md

[dellagustin]

in our player, the UI does not properly react when the control is changed with that button.
seems to be actually working, but with a delay, which indicates the update is coming from the timer, still needs some investigation...

I retested and this seems to be working, but not intentionally

[dellagustin]

Current screenshot after e84bf69

[dellagustin]

Current screenshot after 4fb90eb

[dellagustin]

blocked by #47