Clean some code add CmdDeleteNode.

The undo of the command is not working yet.
Reorganize the application (just all about Graph)

Move data of the graph in the core section.
Expose only buttleData in QML (and still the cmdManager...).
Merge branch 'develop' of github.com:buttleofx/ButtleOFX into develop

Update CmdDeleteNode to delete all his connections.

Conflicts:
	buttleofx/core/graph/graph.py
	buttleofx/core/undo_redo/commands/__init__.py
	buttleofx/core/undo_redo/commands/cmdCreateNode.py
	buttleofx/gui/graph/connection/__init__.py
	buttleofx/gui/graph/graphWrapper.py
	buttleofx/gui/graph/qml/Clip.qml
	buttleofx/gui/graph/qml/Graph.qml
	buttleofx/gui/graph/qml/Node.qml
	buttleofx/gui/graph/qml/Tools.qml
	buttleofx/main.py