Add support for custom Dart entrypoints

[swift-kim]

Usage:

In main.dart,

void main() {
  // I'm not a real main!
}

@pragma('vm:entry-point')
void anotherMain() {
  runApp(MyApp());
}

In runner.cc (native app),

bool OnCreate() {
  dart_entrypoint_ = "anotherMain";
  ...

or in App.cs (.NET app),

protected override void OnCreate()
{
    DartEntrypoint = "anotherMain";
    ...

Caveats:

• The app's main Dart file must declare main() even if it's unused. It's a requirement by the Dart compiler but not the flutter-tizen tool.
• The use of custom entrypoints may break app builds if the app depends on one or more FFI-based plugins. This is supposed to be a very rare scenario.

Depends on flutter-tizen/engine#152.

Closes flutter-tizen/engine#111.

[swift-kim]

Public methods such as SetEntrypoint() may be added later. The current support is experimental.

[bbrto21]

I tested this using .Net host and it works.