Create ViewBuilder struct

[TomSteu]

In the main crate of gtk-rs, most widgets have a builder struct such as:

let builder = gtk::TextViewBuilder::new();
/* ... setup  */
let text_view = builder.build();

which are missing in the sourceview crate alltogether. This is horribly inconsistent. The builders appear (to me) to be auto-generated, would it be possible to provide such structs in the next release?