Add --enable-multi-window option and refactor runner code

[hbatagelo]

This pull request is the framework counterpart of canonical/flutter-engine#1. It introduces the --enable-multi-window command-line option to flutter tools and updates the runner code of the reference application to align with the refactored Windows embedder.

Updated Steps for Running the Reference App

Prerequisites

• Build a custom engine using the refactored embedder
  (The following assumes the engine is built in debug mode)

Run with flutter run --enable-multi-window

cd <path_to_flutter>\examples\multi_window_ref_app
flutter run --enable-multi-window --debug --local-engine-src-path <path_to_custom_engine> --local-engine host_debug_unopt --local-engine-host host_debug_unopt lib/main.dart

Build and run manually

1. Build the application:

cd <path_to_flutter>\examples\multi_window_ref_app
flutter build windows --debug --local-engine-src-path <path_to_custom_engine> --local-engine host_debug_unopt --local-engine-host host_debug_unopt lib/main.dart

2. Set environment variables and run the executable:

set FLUTTER_ENGINE_SWITCHES=1
set FLUTTER_ENGINE_SWITCH_1=enable-multi-window=true
.\build\windows\x64\runner\Debug\multi_window_ref_app.exe

[mattkae]

Working well for me!

[robert-ancell]

LGTM