Backport: Contextmenu does not close after menuitem is clicked (CMS7-6844)

The context menu of the CMS tree is not closed when a menu item is clicked. Only after clicking somewhere on the page (usually by clicking on a field in a dialog that was opened) the context menu is hidden.

Reproduction path:

• click on the little arrow in the CMS tree to open context menu
• click on a menu item that opens a dialog
• in the background you can see that the context menu is still visible
• click on a field in the dialog
• the context menu will disappear

For correct event handling the context menu should be closed after it is clicked. This is now done by a click on the DOM which is a workaround that does not work in all situations, for example, clicking on a flash object in IE.