Linux: fix frameless window & background color

[jpnurmi]

This is a replacement of #248 and implements support for frameless windows and background colors in a way that it doesn't draw over HdyWindow's shadows nor needs any redraw tricks.

[jpnurmi]

Before

The background was not drawn at all unless the window was made frameless.

Normal	Frameless

After

The background color and the window decorations are disconnected. You can set a background color without making the window frameless. As a bonus, the background color works for Handy windows too.

Normal	Frameless	Handy

main.dart

import 'package:flutter/material.dart';
import 'package:window_manager/window_manager.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await windowManager.ensureInitialized();
  // await windowManager.setAsFrameless();
  await windowManager.setBackgroundColor(Colors.white);

  final backgroundColor = ValueNotifier(Colors.white);

  runApp(
    MaterialApp(
      home: Scaffold(
        backgroundColor: Colors.transparent,
        appBar: AppBar(
          title: Text('window_manager_example'),
        ),
        body: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceEvenly,
              children: [
                for (final color in [Colors.red, Colors.green, Colors.blue])
                  ElevatedButton(
                    style: ElevatedButton.styleFrom(backgroundColor: color),
                    child: Text(color.value.toRadixString(16)),
                    onPressed: () {
                      backgroundColor.value =
                          color.withOpacity(backgroundColor.value.opacity);
                      windowManager.setBackgroundColor(backgroundColor.value);
                    },
                  ),
              ],
            ),
            ValueListenableBuilder<Color>(
              valueListenable: backgroundColor,
              builder: (context, color, child) {
                return Slider(
                  value: color.opacity,
                  onChanged: (value) {
                    backgroundColor.value = color.withOpacity(value);
                    windowManager.setBackgroundColor(backgroundColor.value);
                  },
                );
              },
            ),
          ],
        ),
      ),
    ),
  );
}