workbench: add text-button class to menubutton.
snippets: add ginterface snippet for c and chdr languages.

This will create the scaffolding for a new GObject interface based on
the file name. Name files like "foo-bar.c".
source-view: thar be dragons
snippets: use function prefix based on filename
GbDocument: add simple document interface

The goal here is to have the "buffers" behind various tab implementations
implement this and then we can create widgets for them dynamically.