Disabled by default, to enable use
`fl_dart_project_get_ui_thread_policy`:
```cpp
  g_autoptr(FlDartProject) project = fl_dart_project_new();
  fl_dart_project_set_ui_thread_policy(project, FL_UI_THREAD_POLICY_RUN_ON_PLATFORM_THREAD);
```

With this PR, the UI isolate now runs on platform thread, which allows
for synchronous calls to Gtk, as well as synchronous calls from Gtk to
Flutter. For example something like this is now possible:

```dart
typedef GtkWindow = Pointer<Void>;

void setWindowPosition() {
  final library = DynamicLibrary.executable();
  // Hypothetical function to get pointer to GtkWindow
  final getMainWindow = library
      .lookupFunction<GtkWindow Function(), GtkWindow Function()>(
        'get_main_window',
      );
  final gtkWindowResize = library.lookupFunction<
    Void Function(GtkWindow, Int32, Int32),
    void Function(GtkWindow, int, int)
  >('gtk_window_resize');
  final window = getMainWindow();
  gtkWindowResize(window, 200, 200);
}
```

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [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.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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