Rework Chromecast behavior

Problem to solve

Rework the current chromecast behavior.

Intended users

LibVLCSharp.Forms users.

Proposal

Current behavior:

• Checks for chromecast only when chromecast button is pressed.
• Chromecast button animates during search.

Future behavior we want:

Default behavior:

• When MediaElement is created, look for chromecasts immediately.
• If found, show cast button.
• If cast button pressed and only 1 chromecast found, connect to it automatically, else show a popup to choose.
• Update the cast button when a chromecast is added (or removed) to the network after the MediaElement creation.

Customization:

• Allow not to look for chromecasts upon MediaElement creation (extra bindable property on MediaElement I guess).
• Allow to hide cast button (done already).

Documentation

Document how to disable the automatic chromecast search.