Fixed: infinite loop clicking on a pull down menu button.

This was caused by the menu opening up below the button. When the menu manager examined this event it saw a click outside of the menu and immediately closed the just opened menu. Then it put the opening event back on the event queue, causing the pop up button to try to open its menu again.

The fix ignores the opening event for purposes of detecting clicks which should close the menu.

Refs #1833.
New: close pull down menu on second click on its button.

Previously, if a pull down menu was brought down from a pop up button in pull down mode, clicking the button again would close and reopen the menu.

With this fix the behaviour becomes like in Cocoa, and clicking the button a second time simply closes the menu.