Fix up CSS classes for MenuItem subclasses.

Makes it possible to modify L&F of PopupMenuItem by using the dijitPopupMenuItem class.
Also fixes the disabled CheckedMenuItem to be grayed item, like a disabled MenuItem.

Fixes #17045.
fix return value of Dialog.show()/hide() in no-op case, fixes #17100
fix spurious test case failure, refs #17207

mouseMoveAt(TreeNode.domNode) on an open TreeNode moves the cursor
below the associated Tree row.   Specifically, it moves halfway down that TreeNode's
list of visible descendants.    So the test was supposed to be moving the cursor
to the Africa row but instead moved to Kenya.
move mouse tests to Tree_mouse.html, refs #17207