FIX: allow re-shown Qt windows to be re-destroyed

When a Qt window is closed we do not strip off the Qt components and it
is technically possible to re-show it.  However, the latch we use to
detect re-entrant destruction does not get reset so the figure can not be
re-destroyed.

Author: tacaswell

lib/matplotlib/backends/backend_qt.py

@@ -602,6 +602,7 @@ def start_main_loop(cls):
                 qt_compat._exec(qapp)
 
     def show(self):
+        self.window._destroying = False
         self.window.show()
         if mpl.rcParams['figure.raise_window']:
             self.window.activateWindow()