Implement UIA property change event

[mwcampbell]

Once #6 is resolved, raise UIA property change events at appropriate times. Since UIA properties don't always directly map to AccessKit fields, there will probably need to be some ugly special cases here.

[mwcampbell]

#49 includes a generic implementation of this. It avoids ugly special cases by calling property getters on the old and new versions of a node. This might not be the most efficient possible implementation, but we'll handle that problem if it actually comes up. I'll close this issue once I've expanded this generic implementation to cover pattern properties as well.

[mwcampbell]

#79 finishes this.