* 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.
Přemysl Eric Janouch