…support out of tree LinuxWindowingOwners (flutter#188917)

## Problem
While authoring https://github.com/mattkae/layer_shell.dart, I realized
that the Linux Windowing platform had a few major problems when it came
to extending it for out-of-tree windowing owners:

1. 3rd party windows could _not_ register in the `_windows` or `_views`
maps, which meant that they could never be parents. This is invalid, as
other windows (e.g. layer shell windows) may indeed want to be valid
parents and open popups. For example, a top panel may want to open a
sound control dropdown.
2. I wanted to extend `RegularWindowLinux`, but that was unfortunately
not possible because the regular windows realize themselves by the time
the constructor returns. Gtk layer shell windows require that the window
become layer shell _before_ it is realized. We could have added an odd
flag to the regular window, but that felt like duct-tape over a larger
issue whereby an external author could NOT do anything sufficiently
complex with the current API.

## What's new?
- Created a `GtkWindowRegistrar` that can be used to register and
unregister 3rd party windows

Associated PR: mattkae/layer_shell.dart#1

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.