Druid: Have a sane default minimum window size

[cmyr]

Currently by default windows can be made arbitrarily small. I think there should be a default minimum size in the 400x400 range, and if you want your window to be resizeable below that you need to explicitly set the minimum size to something smaller, using WindowDesc::with_min_size.

Just as a note: you can set a minimum size for a window, and I think this should be set for most actual applications? I actually think this should have reasonable defaults, and if you want to be able to resize windows to be very small you should have to specify that manually, but that's also a separate issue. 😁

_Originally posted by @cmyr in https://github.com/linebender/druid/issues/1297#issuecomment-707235539_

[totsteps]

I am in favor of adding a minimum default size to WindowConfig.

When adding a minimum size to a window config either via WindowConfig::default() or WindowConfig::with_min_size, I noticed a small issue with the launched window not respecting the minimum size!

Let's say I add a min-size of 600x600, whereas the launched window has a min-size of 500x400 on macOS!

[cmyr]

@totsteps that sounds like a separate bug, would you like to open an issue for it?

[totsteps]

Filed in #1322.