Show XML tree view as a new icon on the activity bar

[Otiel]

Since the April 2018 update, extensions can now add a new icon to the activity bar. I suggest moving the XML tree view from the Explorer pane to a new pane.

Better even, add an option to choose between having the XML tree view on the Explorer pane or on a separate pane. But to provide such option, we would have to wait for https://github.com/Microsoft/vscode/issues/48704.

[DotJoshJohnson]

I like this idea. I'm going to backlog it for now and circle back around when I have a clearer picture in terms of how to make this optional while allowing reuse of the existing data provider. The parts all seem to be there, it will just take a little thought.

Thanks!