Fixes https://github.com/flutter/flutter/issues/57595

Fixes https://github.com/flutter/flutter/issues/54857

These aren't used by the model so are better managed outside of it.

Match the style used in the external plugins and more carefully vet the incoming
arguments.

Simplify FlDartProject by removing the path arguments - we'll assume that it's
in the standard bundle layout and add new methods later if we need to support
other cases.

This stops events and platform messages generating errors when the engine has
failed to initialize.

The embedder.h API layer is an implementation detail of the desktop
embeddings, not part of the public API surface, so should not be part of
the public symbol list for those libraries.

These are removed in destructors.

This reverts commit a095cc23.

* Add mock implementations of the Flutter embedding API and EGL
* Add tests for FlBinaryMessenger, FlBasicMessageChannel, FlMethodChannel

* Add FlTextInputPlugin

* Add FlPluginRegistry and FlPluginRegistrar

* Add FlMethodChannel, FlMethodCall, FlMethodResponse and FlMethodCodec

* Add FlJsonMethodCodec

* Add FlStandardMethodCodec

Make FlBasicMessageChannelResponseHandle a GObject to detect unresponded and double responded messages. (#18514)

This is more consistent with the Dart code.