Implement rectangle select

[jpmonettas]

Moving a bunch of nodes gets pretty annoying, the only way of doing it so far is by selecting multiple nodes by holding CTRL and clicking every node.

Currently clicking and dragging the mouse on the background drags the diagram so we will need to introduce the idea of multiple pointer tools (dragging, selecting, etc) or hold a key while dragging to tell it you are interested in selecting and not dragging. I think the first one is more standard.