Add UI for switching to open terminals

[aminya]

It would be nice if we add a dropdown that shows all the open terminals and allows to switch to them (bring them to focus), by clicking on the drop-down entries.

This is a little similar to VsCode's terminal dropdown, but the difference is that I want the terminals to be kept in separate Panes (non in one pane). We just need to focus on them.

We can use Terminus's terminal manager for this work. That one also solves the issue of consistent dock buttons (closing/opening terminals quickly). This is actually a better approach than the dropdown. It will be nicer and take less space.

https://github.com/bus-stop/terminus/blob/master/lib/status-bar.js

Currently, I keep all my terminals in the bottom dock, but it is hard to know which open project has a terminal already open. When I close the dock, I tend to forget that there has been a terminal open.

[aminya]

I really like the Terminus UI. I wonder how hard would it be to port it to our package. #60