Extend get_layout to handle active and passive clauses

[gleiss]

Currently get_layout uses the node-id to determine whether it should display the node or not.
We want to change this to:

• if the passive-time is smaller or equal the history_state, display the node
• depending on whether the active-time (if it exists) is smaller or equal the history_state, display the node as passive node or as active node (passive nodes should be in the background, I expect that we are mainly interested in the active nodes, and only look at the passive nodes from time to time)