## Summary of the Pull Request

This PR adds support for renaming windows.

![window-renaming-000](https://user-images.githubusercontent.com/18356694/113034344-9a30be00-9157-11eb-9443-975f3c294f56.gif)
![window-renaming-001](https://user-images.githubusercontent.com/18356694/113034452-b5033280-9157-11eb-9e35-e5ac80fef0bc.gif)


It does so through two new actions:
* `renameWindow` takes a `name` parameter, and attempts to set the window's name
  to the provided name. This is useful if you always want to hit <kbd>F3</kbd>
  and rename a window to "foo" (READ: probably not that useful)
* `openWindowRenamer` is more interesting: it opens a `TeachingTip` with a
  `TextBox`. When the user hits Ok, it'll request a rename for the provided
  value. This lets the user pick a new name for the window at runtime.

In both cases, if there's already a window with that name, then the monarch will
reject the rename, and pop a `Toast` in the window informing the user that the
rename failed. Nifty!

## References
* Builds on the toasts from #9523
* #5000 - process model megathread

## PR Checklist
* [x] Closes https://github.com/microsoft/terminal/projects/5#card-50771747
* [x] I work here
* [x] Tests addded (and pass with the help of #9660)
* [ ] Requires documentation to be updated

## Detailed Description of the Pull Request / Additional comments

I'm sending this PR while finishing up the tests. I figured I'll have time to sneak them in before I get the necessary reviews.

> PAIN: We can't immediately focus the textbox in the TeachingTip. It's
> not technically focusable until it is opened. However, it doesn't
> provide an even tto tell us when it is opened. That's tracked in
> microsoft/microsoft-ui-xaml#1607. So for now, the user _needs_ to
> click on the text box manually.
> We're also not using a ContentDialog for this, because in Xaml
> Islands a text box in a ContentDialog won't recieve _any_ keypresses.
> Fun!

## Validation Steps Performed

I've been playing with 

```json
        { "keys": "f1", "command": "identifyWindow" },
        { "keys": "f2", "command": "identifyWindows" },
        { "keys": "f3", "command": "openWindowRenamer" },
        { "keys": "f4", "command": { "action": "renameWindow", "name": "foo" } },
        { "keys": "f5", "command": { "action": "renameWindow", "name": "bar" } },
```

and they seem to work as expected