implement focusable attribute for Window

[lunixbochs]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

if focusable is false, a window won't take focus, even if you interact with it
this is very useful for always-on-top floating utility windows

[kchibisov]

How keyboard input generally works with such windows or it doesn't and you have mostly pointer input?

[lunixbochs]

Yeah, they don't take keyboard focus, which is a feature - it allows you to interact with the overlay window without taking focus away from the current app. Imagine an on-screen keyboard, for example, we need to be able to click the keyboard buttons but have keyboard input still go to the app in the background.

[madsmtm]

macOS impl looks fine apart from the duplicated state. Requested changes because I'd like to see more docs, but am otherwise fine with the API.

[lunixbochs]

@madsmtm rebased and addressed feedback, see last two commits in the force push

[madsmtm]

Thanks, looks fine, only uncertain about is_focusable return value.

[madsmtm]

Thanks, I'll leave this open for a week or so (please remind me if I forget) for the other maintainers to comment on X11 and Windows impls (I've reviewed them myself, but it's hard for me to know if they're semantically correct).