Revise or restructure menu, menubar, and menubutton patterns to clarify uses of menu

[mcking65]

In issue #261, the discussion surfaced confusion over what a "menu" is. This is not the first time such confusion has arisen.

The current patterns combine menu and menubar into a single design pattern because a menubar is simply a visually persistent menu. However, while the current organization of information is good at avoiding redundancy, it it can be confusing to readers who approach the document with different ideas of what a "menu" is. The negative impacts of the current organization are magnified by the fact that menus are much more common than menubars.