[windows] Use existing window proc delegation & HWND retrieval APIs from Flutter

[alexmercerind]

• feat: expose FlutterWindowClose.destroyWindow for customized usage.
• fix: use flutter::PluginRegistrarWindows::RegisterTopLevelWindowProcDelegate for proc delegation & improve HWND retrieval.
  • Refactored Windows implementation use make use of flutter::PluginRegistrarWindows::RegisterTopLevelWindowProcDelegate instead of changing GWLP_WNDPROC. This gives other plugins opportunity to delegate/listen to window messages as-well.
  • Replaced GetActiveWindow with public API present in Flutter already. GetActiveWindow can result in unpredicted behaviors.
  • Improved code structure & followed Google Style Guide for C++.

https://github.com/zonble/flutter_window_close/blob/79e358db9e1edaa9dc147a7e31d73b0b3ba02cdb/windows/flutter_window_close_plugin.cpp#L73-L75