linux/x11: Fix closing of GPUI windows not working (#13201)

This fixes everything but the main Zed window (GPUI examples, prompt
library, etc.) not being closable by clicking on the X in X11.

We had a dangling reference before: we would remove the window from the
X11 state, but GPUI itself would still have the window in its
references.

In order to fix this we have to call `window.close()`, which ends up
calling `cx.remove_window()`, which removes the reference.

That in turn then causes the reference to be dropped, which cleans up
the X11 state for the window.

Release Notes:

- N/A