use close rather than destroy

It looks like the main window is being destroyed before the dialogs have been freed. Using close seems to have the same effect but lets GTK destroy everything gracefully?

test/gui/dialogs.jl

@@ -25,7 +25,7 @@ error_dialog("Here's an error", main_window; timeout = 0.25)
 sleep(1.0)
 GC.gc()
 sleep(1.0)
-destroy(main_window)
+close(main_window)
 
 end
 
@@ -52,7 +52,7 @@ sleep(1.0)
 
 GC.gc() # ensure GtkDialog is really gone before main_window
 
-destroy(main_window)
+close(main_window)
 
 end