setBackend() with multiple backends

This MR adds additionnal setBackend mechanisms:

• Operator::setBackend() with a list of backends: select the first one available for the Operator;
• GraphView::setBackend() with a list of backends: select the first one that is available for all the nodes in the GraphView;
• GraphView::setBackends() with a list of backends: select the first backend that is available, independently for each node in the GraphView.

Added SinglePassGraphMatching::buildQueryFrom(), which allows to build a query that matches the provided graph.