ButtleData is the owner of the current nodes.

The 3 current nodes :
- currentParamNodeName
- currentSelectedNodeName
- currentViewerNodeName
(and 3 other : their wrapper)
Reorganize ButtleApp.

ButtleApp receive the signal from QML that we create and delete nodes and connections.
So now, GraphWrapper is just a map, a view of a graph.
Update param and viewer when we delete the current node from the graph.
Merge branch 'develop' of github.com:buttleofx/ButtleOFX into develop