Adds links state for terms-based model.
Adds links state for terms-based model.

``TermsAndLinksState`` represents the foundation of the new term-based
model. Terms are selected first in the process of marking or unmarking
links as selected.

git-svn-id: 6391b798f55e3e4f5184bc6d113ebedc82009d5e@3718 1fbec0df-f6b4-4bca-857c-de90862b47bd

gotgenes/BiologicalProcessNetworks