Flutter-desktop-embedding: [window_size] Allow setting min/max sizes

[zhujintao]

[stuartmorgan]

Currently the way you would do this is using NSWindow APIs in the Runner code (or set it directly in the XIB).

This is something the window_size prototype should support though, as it's come up several times.

[stuartmorgan]

It's going to require some thought to figure out how to make this work on Windows. Unlike macOS and GLFW, where it's a settable property, on Windows it's implemented by replying to a window message that's send during resize (WM_GETMINMAXINFO), so we'll need to figure out how to bridge between a plugin and the window class, and maybe set up a generic message handling delegation system.

[stuartmorgan]

Now implemented on all platforms.