Add support for side panels

[thernstig]

Description of the new feature/enhancement

Tools such as Visual Studio Code or even MobaXterm support side panels. Imagine something like:

• The red lines showcase the various profile names
• The blue lines indicate the current open connections to each profile. Be that tabs or side panels. The UX for tabs vs. sidepanels needs to be figured out.

It would be neat if Windows Terminal had an optional side panel support. Suggested keyboard shortcut:
ctrl+b

Proposed technical implementation details (optional)

No idea.

[zadjii-msft]

This kinda just seems like a case for #997 and #4000. 4000 has a lot of linked threads to various UI elements that might be used in panes. I guess my question would be more specifically, what do you want surfaced in that pane?

• A "List of Profiles" pane seems... just about as useful as the dropdown.
• A "List of tabs/panes" pane doesn't seem like a terrible idea, though I think there's some overlap with the ideal form of the tab switcher there
• Something else/?

[thernstig]

As visualized in the original post, the blue lines should indicate open connections (be that tabs or panes). And that the red lines indicate to which profile the connection is open.

This is thus another view of the current tabbed and pane layout, which many users prefer, especially people coming from MobaXterm or similar tools. The use case is a UX use case for a more advanced way of visualization all opens profiles + connections than what tabs can do.

We can also map this 1-to-1 with how VS Code has an OPEN EDITORS functionality. It is a more advanced form than tabs to visualize all open editors.

Imagine having 30 open tabs. Moving between these tabs is easier in a side panel like above, than trying to find the tabs. It is even harder if having panels within tabs, to find exactly where you are.

I am not an UX expert, but I am sure an UX expert could fill in even more why a side panel is in many ways superior to tabs.

Aside: I used the term Panel on purpose, since it is the convention used by Visual Studo Code. Even though you render it in a "pane" (as described in #997), I think the term Panel is better.

[zadjii-msft]

Huh. So, basically just, vertical tabs (ala #835), but you also want to group the list of tabs, by what profile is open in them/?

[thernstig]

Like MobaXterm (though of course with the much nicer Windows Terminal UI):

[zadjii-msft]

Wait is the more important thing the grouping? Or just the vertical list of open tabs/?

Because if it's grouping, the new tab menu supports that as of #1571 merging in like, 1.17:

That might be quick solution here.