Release 0.12

This is a maintenance release that included major refactoring and
simplification of the design of the graph management core, and the
addition of corresponding design documentation. This refactoring led to
a number of small improvements:

• It is now possible to select, move, and delete point nodes.
• Copy to clipboard now correctly clips the graph bounds again.
• Removed the grow/clip drawing area feature to simplify the design.
• Fixed small problems related to the addition and copying of nodes.
• Fixed a bug related that prevented redoing the pasting of edges.