Keep track of when the extension is running an experiment

[rogermparent]

Related to #230 and #231, and loosely to #261

In order to add UI elements that conditionally change when there is a currently running experiment that we can stop, we'll need to add the ability to track when an experiment is currently running.

This can be deceptively complicated, as with the IntegratedTerminal we currently cannot directly know when a command is done executing. We'll need to solve this with something like a Task+Pseudoterminal before solving this, so much so that an improved way to run terminals with activated environments is 99% the solution to this Issue, with the rest being a public boolean field.

There's also the consideration of keeping track of when external experiments are running, and while that could be a useful thing to know, there isn't a much we can do with that information that isn't already taken care of by knowing when the repo is locked unless we can find the pid of the running experiment, which I don't believe we can with dvc's current behavior.

[mattseddon]

Related to #267.

[mattseddon]

@rogermparent I think we've handled this successfully now, feel free to re-open if not.