animator: Allow retrieving type property type from a key

animator: Allow retrieving type property type from a key

When asking a key for its target value we also ask the developer to pass
in an initialized GValue - but we don't make it easy to know the type of
the GValue. A developer has to ask the GObject class for the GParamSpec
and then initialize the GValue, instead.

Since we know the type of the GValue we should provide a getter for it.

We should also allow developers to throw at us GValue with compatible and
transformable types.

Finally, all the accessors should be constified.