Add missing _new

As encountered previously you have a habit of declaring a _new without actually implementing a _new

For your coding style this is fine because g_object_new works as expected (and GIR based languages also) but for those of us writing Vala or C we end up with an unexpected linker error

Note this doesn't add any missing _new just implements those that were already declared

So in the cause of EmptyState we can avoid hacks like this

var state = Object.new(typeof(Dazzle.EmptyState)) as Dazzle.EmptyState;

because the linker error is fixed but

var tree = new Dazzle.Tree();

is still an error for calling a protected constructor because there is no dzl_tree_new

Branch name is a little misleading