Add toolTip to SC.MenuPane items.

Added the ability to set tooltips on menu items.
Clean up SC.MenuItemView properties.

There was some inconsistency with the item*Key family of properties.
Remove the dependent keys from some SC.MenuItemView properties.

Since these properties will be updated manually based on the configured
key, there is no reason to have a static dependent key specified.
Add unit test for previous SC.MenuItemView changes.