I noticed this last week, but forgot to file. If you have a pair of
splits, and `exit -1` the first, you can't use `enter` to restart it.

This PR fixes that. Basically, `TerminalPage` registers it's
`_restartPaneConnection` handler when it makes a new `Pane` object. It
registers the callback straight to the `Pane`. However, when a `Pane`
gets split, it makes a _new_ `Pane` object, and moves the original
content into the new pane. `TerminalPage` however, would never hook up
its own callback to that newly created pane.

This fixes that.

(cherry picked from commit 9b986a1)
Service-Card-Id: 90584595
Service-Version: 1.18

Instead of `Pane` hosting a `TermControl` directly, it now hosts an
`IPaneContent`. This is an abstraction between the TermControl and the
pane itself, to allow for arbitrary implementations of `IPaneContent`,
with things that might not be terminals.

## References and Relevant Issues

* #997
* #1000

## Detailed Description of the Pull Request / Additional comments

This PR by itself doesn't do much. It's just a refactoring. 
- It doesn't actually add any other types of pane content. 
- It overall just tries to move code whenever possible, with as little
refactoring as possible. There are some patterns from before that don't
super scale well to other types of pane content (think: the `xyzChanged`
events on `IPaneContent`).
- There's a few remaining places where Pane is explicitly checking if
its content is a terminal. We probably shouldn't, but meh

There are two follow-up PRs to this PR:
* #16171 
* #16172 

In addition, there's more work to be done after these merge:
* TODO! issue number for "Replace `IPaneContent::xyzChanged` with
`PropertyChanged` events"
* TODO! issue number for "Re-write state restoration so panes don't
produce `NewTerminalArgs`"

## Validation Steps Performed

* It still launches
* It still works
* Broadcasting still works
* The weird restart connection thing from #16001 still works

## PR Checklist
- [x] Closes #997


## other PRs
* #16170 <-- you are here 
* #16171
* #16172