Use recursive mutex for GUI window handling

[catmeow72]

This avoids a crash when opening a window in onClose.

[catmeow72]

Note that this does not yet seem to allow to actually open a window in onClose, but at least the attempt doesn't crash.

[IntegratedQuantum]

I don't like recursive mutexes, and this PR is a good example why they can lead to bugs.
With your changes you are appending to the list while iterating on it, which could result in a use after free in the worst case.
And of course the list is cleared after the iteration is done, which is why the newly added element is discarded without further consideration.