Modal windows / dialogs

[Enyium]

I propose to add a new constructor function to generated Window/Dialog component structs besides new(): new_modal() (with_owner() would be a terminology that's too Windows-specific). It would take a reference to another type implementing ComponentHandle as parameter. Because of lack of support from winit, on other OSs than Windows, this currently simply would be ineffective. On Windows, winit provides WindowAttributesExtWindows::with_owner_window(), which you'd need to use. Whenever the owned window is shown, the owner window must be disabled (with WindowExtWindows::set_enable()). When the owned window is hidden, the owner window must be enabled before hiding it.

[ogoffart]

Some work in progress in #8135
The idea is to have a Window::show_modal that takes another Window (or none of application modal)
That PR implements the feature with the Qt backend, but the winit implementation is more complicated as there is no direct equivalent.
I've tried on X11 but that doesn't seem possible at this point. So this will require us to do some changes to winit to enable modal dialog on all platform.

[ultimaweapon]

With the latest Winit it is actually possible to implement a modal dialog on all platforms (including X11 and Wayland). We have done it recently on our project:

X11 and Wayland: https://github.com/obhq/obliteration/blob/main/gui%2Fsrc%2Fui%2Flinux%2Fmod.rs#L33
macOS: https://github.com/obhq/obliteration/blob/main/gui%2Fsrc%2Fui%2Fmacos%2Fmod.rs#L28

For Windows we have not implemented yet but I known it is possible since I was working with Win32 a lot in the past.