gtk4 fcmp: Destruction of quit dialog does not allow opening a new one
New version for S3_0 only - handle gtk4 modpack installer in that branch too.
gtk3- and gtk2- guis aren't actually affected. They get the correct pointer nullified via gtk_widget_destroyed() callback. Should not touch those, especially in stable branches.
- New version of the patch, that applies to all branches, with changes only to gtk4 gui
Noticed on master gtk4 fcmp, other branches and fcmp guis likely affected.
quit_dialog_destroyed() nullifies just a local variable. It does nothing to the one in quit_dialog_callback() that controls whether a new quit dialog can be opened or not.