extract webview extension

[akosyakov]

Webview is an abstraction to load remote content in the safe way. #6465 brings its support to the plugin system only. It is a proposal to extract it to own extension to reuse by Theia extensions. There are many benefits beside security:

• links to localhost resolved to remote centrally for the whole application by ExternalUriService.resolve
• good integration with the shell out of the box, storing/restoring widget
• focus tracking, styling, keyboard propagataion, scrolling preservation between drag and drops and so on.

The good candidate to use it will be mini-browser, which is suffering from similar security and functionality issues. Generally whenever someone thinks he needs to load remote content with iframe, they should better to use webviews.

cc @svenefftinge

[progrium]

I'm into this. Tell me how I can help.