Refactor widget properties

[jonathanstowe]

The design decision to make the gtk widget properties r/w accessors using Proxy is sound, however it results in a lot of very similar boilerplate which at best makes implementing all the properties rather tedious, but is also a maintenance and testing issue.

I'd like to re-implement the properties with a trait such as gtk-property which provides the implementation of the properties in a very similar way to AccessorFacade does (just adjusted to take into account the structure of the widget classes.)

I'd say this was relatively high priority as it would be better to do before implementing too many more properties and/or widgets.