Better visual feedback for panels

[evidolob]

Description

When Theia works on slow internet connections, it some times hard to understand that Theai performs some background work(fetch files from server, communicate with plugins, etc.) when panels open(like file tree).
Now we have some issues like:
#5959 or #6195

I propose to have progress indicator similar to git/scm for all panels (I think we can do it on shell level),
and make sure that plug-in API implemented correctly, so plug-in's can use it to have same ux/ui as in VSCode

[evidolob]

cc @benoitf @slemeur @akosyakov

[akosyakov]

It makes sense from user perspective, but probably not for each panel, but for whole area? E.g. if language services are fetching something, it would be enough to show a bar at the top of whole main area, not for each editor. It is just too noisy. The same for side areas, the view container can have it at the top, but showing for each view would be too noisy again. I think it is the same in VS Code.

I cannot check code right now, but it should be possible already for view constrainers contributed by the plugin system, maybe it is not accessible via theia.d.ts though, @AlexTugarev can correct me. He did work to align the progress service with VS Code behavior last time.

[akosyakov]

It would be good if someone breaks down this issue in actionable items, i.e. for each existing view we should decide which work can take a while and then we should wrap such work into progress to display for the corresponding view container.