Support for Web and Desktop about application_icon

[ueman]

macOS support should be pretty straight forward as probably most iOS code can be reused.
Web support can be added by reading the web manifest (manifest.json).
I suppose Windows should be pretty straight forward too. Just read the windows/runner/resources/app_icon.ico file. I don't own a windows device, so I would love a PR for this.

I'm not sure about linux though.

[ueman]

Window

Maybe this helps for windows:
https://pub.dev/documentation/win32/latest/win32/LoadIcon.html

A handle for the first parameter can probably be obtained via
GetModuleHandle(nullptr). See https://stackoverflow.com/questions/1749972/determine-the-current-hinstance

Also see https://stackoverflow.com/questions/57688782/copy-hicon-hcursor-in-to-byte-array

A windows implementation can then make use of the dartPluginClass

flutter:
  plugin:
    platforms:
      windows:
        dartPluginClass: ...

[ueman]

Linux

See
https://stackoverflow.com/questions/41690822/how-to-set-executable-application-icon-in-linux-based-qt

Platform specific code can also be loaded via dartPluginClass

On linux the icon can probably load in pure Dart code

[ueman]

Web

There's the manifest.json file which contains links to icons. Though there's no guarantee that an icon is available. As a fallback we could check for favicons (which also doesn't need to exist).

All icons can possibly be an svg.