[plug-in] TreeView should be created on registering a TreeDataProvider

[azatsarynnyy]

Currently, Theia adds a TreeView for each registered TreeDataProvider only once - as soon as the frontend app is reached a ready state. It's not correct behavior for two reasons:

• a plugin can register a TreeDataProvider at any time but not only from start entrypoint
• on slow infrastructure a plugin registers TreeDataProvider later than the frontend app goes into a ready state

In any of the cases, plugin's tree isn't shown in a contributed view.

It works perfectly in standalone Theia but the problem is stable reproducible on some slow environment. For example, try the following steps:

• run Eclipse Che with Theia editor on minikube or use https://che.openshift.io
• clone the https://github.com/eclipse/che-theia-samples into workspace
• run the tree-view-sample-plugin on development host
• the comments view is empty

When I add some delay before the front app goes into a ready state then tree is shown in the view. To add such delay open several files and reload the browser's page.

[vitaliy-guliy]

One thing. I suppose TreeView must be created immediately only when ready state of Theia is reached.
Anyway we need to check.