Graph: update clear function

* Call clear function of InternalGraph.
* Clear the instance count.
Graph: add getNbNodes and getNbConnections
pyTest: add testDeleteNode
pyTest: add testClearGraph
Graph: refactoring - rename

* Rename _nodes to _nodesMap.
* Rename functions getNodes() by getNodesMap().
Graph: add getNodes

* GetNodes() functions return a vector of Nodes.
* Note: could easily iterate on result in python binding.
InternalGraph: add getters to connected and unconnected vertices

* Add getConnectedVertices function.
* Add getUnconnectedVertices function.
* Add include to boost/graph/connected_components.
InternalGraph: update removeUnconnectedVertices

Use getUnconnectedVertices function.
Graph: add getters to connected and unconnected nodes

* Add getConnectedNodes function.
* Add getUnconnectedNodes function.
Graph: add deleteUnconnectedNodes
pyTest: add testDeleteUnconnectedNodes
InternalGraph: refactoring - suppress unused typename
removes the embedded python environment from the text plugin
adds callback system before rendering. The callback definition can be done in python
removes useless comment
removes commented code
removes isExpression parameter from Text plugin
corrects unitialized _beforeRenderCallback
adds callback class and its python binding
Merge branch 'develop' into node_callbacks

Conflicts:
	plugins/CMakeLists.txt