This moves the app template more toward being a more generic starting
point for any Flutter application, eliminating some hard-code
assumptions about there being a single window/engine pair that is
directly bound to the life of the application:
- Moves the runloop into its own class, making it capable of servicing
  any number of engine instances.
- Moves the logic for setting up a window containing only a Flutter view
  into a window subclass for ease of re-use.
- Makes quit-on-window-close an optional property. (Long term this
  should be even more generic, like a quit-when-last-window-closes
  option, but this is a short-term improvement that removes the binding
  between the runloop and the window).
- Allows for multiple instances of Win32Window to exist without issues
  relating to the window class registration.

Since there are getting to be a non-trivial number of files associated
with the runner, this moves the source into a runner/ directory, as is
already done on some other platforms.

Note that creating multiple Flutter windows at the same time still
doesn't work correctly even with this change, but this addresses some of
the known issues, and makes it easier to test in the future (e.g., for
debugging engine-level issues with multiple instances).

Fixes #45397

This reverts commit d6b09626.

This reverts commit 8a3bede1.

This reverts commit 66d7a6c2.

* Usage dependency injection

* Review edits

Makes the following changes to the behavior of precache:
- The --all-platforms flags now fetches all artifacts, rather than just
  turning off platform filtering of selected artifacts.
- Explicitly requested artifacts are no longer subject to platform
  filtering. E.g., 'precache --ios' will download iOS artifacts on
  Windows and Linux (but 'precache' without an 'ios' flag will still
  only download iOS artifacts on macOS).
- Desktop platform artifacts now respect the bypassing of platform
  filtering.

Fixes #53272

This will allow experimenting with the remove to string transformer before we're ready to turn it on by default. This doesn't work for web yet since we use dart2js instead of the frontend_server for producing kernel