Add UI for switching to open terminals

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.

Start by reading Terminus's lib/status-bar.js, which the issue identifies as the reference for terminal-manager behavior, then inspect this package's existing terminal and dock UI entry points. Done means users can see open terminals, focus a selected terminal in its separate pane, and use consistent controls to open or close terminals.

Written by the indexing model from the issue text.