related to #15558

* Use the extension descriptions available in the UI thread
* Don't wait for handling extension points until extension host is running
* Create the extension host process only after extension points have been handled
* Send the initial configuration as part of the initData to the extension host

Drop the viewlet part in treeExplorerViewletService

debt - no guessing-caching, better use of gc-signals, command converter using gc-signals, main side heap service