Fix issue #67, also prevent duplicate events on window show/hide.
Fix issue #67, also prevent duplicate events on window show/hide.

kmonsoor/pyglet,mpasternak/pyglet-fix-issue-518-522,arifgursel/pyglet,Austin503/pyglet,mpasternak/pyglet-fix-issue-552,mpasternak/michaldtz-fixes-518-522,Alwnikrotikz/pyglet,mpasternak/michaldtz-fixes-518-522,google-code-export/pyglet,google-code-export/pyglet,mpasternak/pyglet-fix-issue-552,qbektrix/pyglet,Alwnikrotikz/pyglet,cledio66/pyglet,mpasternak/pyglet-fix-issue-552,mpasternak/michaldtz-fix-552,gdkar/pyglet,odyaka341/pyglet,kmonsoor/pyglet,mpasternak/michaldtz-fix-552,Alwnikrotikz/pyglet,shaileshgoogler/pyglet,arifgursel/pyglet,arifgursel/pyglet,Austin503/pyglet,xshotD/pyglet,gdkar/pyglet,google-code-export/pyglet,odyaka341/pyglet,kmonsoor/pyglet,xshotD/pyglet,shaileshgoogler/pyglet,Alwnikrotikz/pyglet,Alwnikrotikz/pyglet,gdkar/pyglet,odyaka341/pyglet,Austin503/pyglet,xshotD/pyglet,qbektrix/pyglet,qbektrix/pyglet,gdkar/pyglet,cledio66/pyglet,mpasternak/pyglet-fix-issue-518-522,xshotD/pyglet,arifgursel/pyglet,Austin503/pyglet,mpasternak/michaldtz-fixes-518-522,mpasternak/michaldtz-fixes-518-522,mpasternak/pyglet-fix-issue-518-522,Austin503/pyglet,arifgursel/pyglet,mpasternak/pyglet-fix-issue-552,odyaka341/pyglet,kmonsoor/pyglet,odyaka341/pyglet,shaileshgoogler/pyglet,shaileshgoogler/pyglet,shaileshgoogler/pyglet,cledio66/pyglet,qbektrix/pyglet,google-code-export/pyglet,gdkar/pyglet,qbektrix/pyglet,mpasternak/michaldtz-fix-552,cledio66/pyglet,xshotD/pyglet,cledio66/pyglet,mpasternak/pyglet-fix-issue-518-522,google-code-export/pyglet,mpasternak/michaldtz-fix-552,kmonsoor/pyglet