[UI] [enhancement] Change right-click menu closing logic

[rbohrer]

In most UIs for most applications, if right-clicking opens a context menu, it is standard to close the menu whenever the user clicks outside the menu. However, the right-click proving menu on the KeYmaera X proof interface only closes when the user clicks an "x" button, which is one of the smaller buttons on the UI and easier to miss. It would be less surprising for new users to follow the more common UI pattern