* Add generic methods for data storage, making it able
to store data in a hierarchy.
* Reimplement ld_diagram_{get,set}_data_for_param on top
of the generic methods.
* Add a "data-changed" signal.
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.
