Mutable struct build

[JFronny]

While attempting to implement #20, I noticed that the java-gtk implementation for this updates callbacks in existing fields.
In java-gi, it seems like a Build instance is required to mutate these, which is why this PR allows creating a Build instance of an existing object.
Another problem I came across is the fact that callbacks are represented as raw MemoryAddress instances instead of lambda types, but I don't know how to fix that.

[jwharm]

Those callbacks don’t work with lambdas because a user_data field is required for lambdas. I’m currently working on a fix in a separate branch, but that will take some more time.

[jwharm]

I think the best way to update callback fields, is to generate a field$set() method for callback-fields. Just remove this check from Record.java around line 79:

        // Don't try to generate a setter for callbacks
        if (type.isCallback()) {
            return;
        }

The Build class is meant as a construction mechanism (the builder pattern) and it works quite well to quickly instantiate GObjects with properties. However, I went a bit overboard and also added a builder to set struct fields. In hindsight, that was not the best idea. You can merge the PR but I'm not sure the struct builder should stay anyway.

[jwharm]

Regarding #20, that one will be a lot easier to implement once I finish the new implementation of callback handling (#15).