Add support for mutable lists

[JFronny]

Some lists in GTK support mutation via append, remove and splice.
This implements the required methods to allow mutation on them.
Additionally, this allows patches to pick implementation classes for generics where appropriate (like StringObject for StringList as its documentation suggests)

[jwharm]

Thank you, this looks really promising!

[jwharm]

Looking good, I will merge this.
Do you mind if I add copyright notices to the new files?

When I have some more time, I will probably change the new java-gi-to-string attribute a bit, so I can use it instead of the custom generated toString methods on GValue, GType and GVariant.

[JFronny]

Do you mind if I add copyright notices to the new files?

I do not mind, feel free to add them.

I will probably change the new java-gi-to-string attribute a bit

I have actually experimented with that a bit, you can find my results here