Save/load data from buttleofx/backup/data.json

2 problems :
	- we can't display computed image after loading a graph
	- the display of connection is approximative
Next important steps :
	- better display for save and load : buttons QML...
	- possibility to save and load from different files
Merge branch 'iss18' into develop

Conflicts:
	buttleofx/main.py
Merge branch 'develop' of github.com:buttleofx/ButtleOFX into develop

Conflicts:
	buttleofx/gui/graph/node/nodeWrapper.py
	buttleofx/gui/graph/qml/Node.qml