Unify watch property methods.

[sjohnsonaz]

Fix increasingly complex watch methods.

• arguments vs argument object
• getter and setter methods
• overriding

[sjohnsonaz]

Module and Widget now have identically functioning watch methods. Currently they can all accept a get and set function.

• Get has both a value and this argument.
• Set has a value and a cancel argument, which may be passed to cancel the set.
• Methods are optimized with different versions depending on existence of get and set.
• Watch methods may be modified. Currently this just simply replaces the original value. We may want chaining later.