Leverage notification progress support for progress API

[bpasero]

With the new notifications comes support for showing long running operations:

We currently have an API where you can show global progress:

One advantage of having this within progress is to free space from the status bar and also to indicate from which extension the progress is reported (because notifications always show their source). Another advantage is that we could provide a "Cancel" button to stop the operation.

We could move this into a notification as a first step and could offer additional APIs to:

• report more detailed progress on the progress bar
• provide additional actions ("Cancel" for example)

@jrieken thoughts?

[Tyriar]

@sean-mcmanus might find this useful for the C++ extension. Related: microsoft/vscode-cpptools#1160

[chrisdias]

@EricJizbaMSFT @fiveisprime this would be great for the Azure extensions which have long running processes creating resources...

[dragonwolf83]

This would be useful for the PowerShell Extension as well.

[jrieken]

Yeah, maybe another ProgressLocation is what we want. Something like ProgressLocation.Prominent and then add a cancellation token to the task-callback. An alternative approach would be to have a progress-message, tho it feels wrong