Add support for INotifyPropertyChanging

[Cologler]

Since EF core required INotifyPropertyChanging, if a type is implemented INotifyPropertyChanging, it should also fire the PropertyChanging event.

[canton7]

Fair request. It opens up a bit of a can of worms around whether we support On{PropertyName}Changing, OnAnyPropertyChanging, etc. We'll also need to duplicate all of the logic to find whether the base class already implements this interface, find an appropriate base class method to call, etc. There's also questions around whether such methods would be passed the old and new values of the property, in the same way that they do for PropertyChanged.

There's also the question of whether we automatically add INotifyPropertyChanging, or whether we get some indication from the user that they want us to implement this interface.

It looks like Fody went the whole hog and implemented all of the above (although they only allow handlers to receive the "before" value). It's a bit of a refactor for PropertyChanged.SourceGenerator, but hopefully not to bad.

[canton7]

Opened #14

[canton7]

Apologies for the delay -- this took a while to implement. I think it's pretty much there though.

[canton7]

Released in 1.0.4