Windows as entities

[fowenix]

What problem does this solve or what need does it fill?

Windows can't be closed from within bevy, #3575 mentions windows as entities and i really like that idea of that, so here is a proposal on how it could be done

What solution would you like?

WindowId is now Entity

Extend Commands with a window method returns WindowCommands similar to how the entity method returns EntityCommands, WindowCommandQueued will implement Command and on write send itself as an event that window backends like bevy_winit will process.

Window should be separated into different components, WindowCursor, WindowCursorPosition, WindowHandle, WindowPosition, WindowResizeConstraints, WindowResolution, WindowTitle, as well as these markers WindowCurrentlyFocused (WindowFocused is taken by a window event), WindowDecorated, WindowResizable.
A world query struct containing all these components and markers should be publicly exposed..

Windows should be spawned like an entity in Commands with a WindowDescriptor component, window backends should query Added<WindowDescriptor> and create the windows for them, while also applying a Window marker confirming they have been created, query Added<Window> or use a WindowCreated event to get newly created windows.

Despawning windows should be done just as entities (as they are now entities), window backends should close/destroy windows their internal component has been dropped, e.g. WinitWindow (wrapper around winit's Window that destroys itself on drop)

A resource called PrimaryWindow contains the entity id of initial window created by WindowPlugin, this replaces Windows::primary

WindowPlugin should change exit_on_close into a enum called ExitCondition with the values OnPrimaryClosed (when primary window is closed), OnAllClosed (when all windows are closed), DontExit (keep app headless)

[Shatur]

I like it so much. Could also solve #3754 because I will be able to such entities myself without winit.

[SpecificProtagonist]

Sounds great.

The marker WindowPrimary should exist on only one window at a time

To guarantee that invariance, maybe it should be a resource holding an entity id instead?