shell/app: Correctly track the window used for the fallback icon

shell/app: Correctly track the window used for the fallback icon

We were not properly tracking the window used for the fallback icon.
This could trigger a crash, as disconnection of the signal handler might
happen on the wrong window, which in turn could cause the icon change
notification to happen on a destroyed ShellApp instance.

Fix this by tracking the window used for the fallback icon. Disconnect
the icon notify callback explicitly for this window only when it is
removed.

Also, just to be extra safe, make sure that the icon is never NULL even
if x11_window_create_fallback_gicon should return NULL for some reason.

Closes: #4436
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2065>