RFC: switch from getindex to getproperty

I realize this will cause a bit of pain (updating the tests and docs strained my fingers too). However, I now believe this is the correct course of action. In the future, I will alias the syntax `obj.title = "something"` and `obj.title[String]` to these methods, but right now that is only partially entirely feasible (via the syntax `obj.title[] = "something"` and a little auto-codegen magic).

I am making this change because I want to use the `obj[:someproperty]` syntax to alias with the GAccessor module, so that you could write `slider[:value] = 1` and that would invoke `G_.value(slider, 1)`, e.g. `gtk_slider_get_set_value`

(I plan to tag this as v0.5.0)