Implement functionality to add/remove sets of nodes to/from the Render Graph

[emanuele3d]

Includes:

• added nodes are just "floating" in the Render Graph. Further steps establish connections to other nodes. Only once a node is declared fully connected it should trigger the refresh of the RenderTaskList. Nodes should be either fully connected or floating, but not in-between.
• it should be possible to add a set of new floating nodes as a batch.
• removal of nodes should trigger the rewiring of the RenderGraph to avoid leaving connectivity gaps.
• it should be possible to remove a set of nodes as a batch. Rewiring takes place for every removed node, but refreshing the RenderTaskList happens only when all nodes in the set have been removed and all rewiring has been done.
• the refreshing of the RenderTaskList should, at this stage, be done by brute force, starting with a new list of ordered nodes, regenerating the internal intermediateList and eventually the renderTaskList.
• this functionality should be made available to modules
• issue #4 - you might want to remove it if it doesn't give you additional information

Does not include:

• addition and removal of entire graphs. It will have to be done outside of the GSOC project.