Fixes problems with SC.MenuPane.

- it will now observe the items array for changes to its contents, so if the items array object itself doesn't change we can still update the menu items
- it no longer shares the default items array object with all instances of the menu pane
- it no longer shares the internal key equivalents hash object with all instances of the menu pane
- removes unused variables
- fully deprecates support for an items object to be an array
- improves jsdocs