Fix non-empty project folder confirmation dialog

[KoBeWi]

A special dialog opens when you try to create a project in non-empty folder. It will also print

ERROR: Attempting to make child window exclusive, but the parent window already has another exclusive child. This window: /root/@ProjectManager@1100/@ConfirmationDialog@1264, parent window: /root, current exclusive child window: /root/@ProjectManager@1100/@ProjectDialog@737
   at: Window::_set_transient_exclusive_child (C:\godot_source\scene/main/window.cpp:951)

The PR fixes it and also makes the dialog reused, instead of created every time.

[akien-mga]

Thanks!