Menu: split out DropDownMenu from ContextMenu

[bill]

There's lots of code in Menu that's only used for context menus. Make separate widgets for drop-down menus vs. context-menus to avoid loading unneeded code.

For backwards-compatibility reasons the ambiguously named dijit.Menu needs to maintain it's old functionality of being a context menu or a drop down menu.

[bill]

(In [24570]) add hints to test, refs #12816

[bill]

(In [24630]) Menu refactor to split out the context menu (with special code for binding to nodes and watching for right-click event) from the plain drop-down menu.

For backwards-compatibility dijit.Menu retains the context-menu functionality, and the plain menu is called dijit.DropDownMenu.

Patch from Akira Sudoh (IBM, CCLA), thanks! Fixes #12816 !strict.

[bill]

(In [24872]) remove unused dependencies, refs #12816 !strict.

[bill]

(In [25392]) Unfortunately, Menu etc. can't extend _WidgetBase instead of _Widget because we lose deprecated methods like attr(). Refs #12816 !strict.