Modify LdDiagram and LdDiagramObject to use it. Trash commit c2403fdcf7885d7da1efa2c0dfb3e294f760ca9c.
Add ld_diagram_remove_selection().
Add an error domain for the purpose of returning errors.
This is required for gtkdoc-scangobj. So far it's much like it's been before, the main differences are that source files are in two directories from now on and the build process has two stages.
