qt: unmap indirection X11 window before reparenting it back to root

Reparenting a mapped window to root causes it to become a toplevel window and can cause window managers to reparent the window themselves to show their decorations.

This could happen in two situations:

1. Theoretically it could happen briefly between WindowReleased and WindowClose.
2. It could trigger a race between VLC and the WM if there is a quick sequence of WindowOpen -> WindowReleased -> WindowClose -> WindowOpen such as when using single instance mode. If the X-server in this situation happens to re-use the XID of the X11 indirection window from the first WindowOpen for the second WindowOpen, this could result in the following race:
   • VLC creates a new X11 indirection window, reparents and maps it
   • VLC reparents it to root
   • VLC closes the connection, freeing the XID
   • VLC opens a new connection
   • VLC creates a new indirection window with the same XID as before and reparents and maps it
   • The X-server sends a MapRequest for the first X11 indirection window caused by the reparent to root to the WM
   • The WM sees a new toplevel window and reparents it. Because the XID used by that reparent request is the same as of the newly created indirection window, the new window ends up getting reparented. This then would result in the indirection window remaining as toplevel window. The exact order might change slightly between the different processes, but as long as the reparent request by the WM is the last one, the issue will trigger.

Closes #25692 (closed)