Make area selection work.

[wdanilo]

Allow selecting multiple nodes by click and dragging a selection area (rectangle).

Specification:

• The rectangle should be created when a dragging action happens on the empty background. The drag-area is already implemented and it is disabled in the graph editor FRP. It is part of the mouse cursor API (the mouse cursor grows during the drag). This should be uncommented.
• Shape intersection should be calculated as bounding box intersections (without caring of rounded corners of both selection area and nodes). Probably an API exposing node bounding-box should be added.
• While the selection rectangle is visible, nodes that are getting selected should get selected interactively (during the drag), but the selection should not be persistent until the dragging has ended. For example, we start dragging the selection area over 10 nodes, and 10 nodes get selected. Then we still drag the same selection area, but we shrink it, and only 3 nodes are selected now (7 other nodes lose selection).

[MichaelMauderer]

• Should also reflect keyboard modifiers for different selection nodes. (e.g., adding selection)
• Graph editor will hold a set of node ids of the temporary selected nodes and keep track of them.