Node selection

Select multiple nodes by holding down Shift and clicking on the nodes.
Deselect a node by clicking on selected node while holding down Ctrl/Cmd.
Select many nodes at once by dragging a selection box around them using Shift.
If Shift is still down when selection box is released, the new selection will be added to the existing selection.
If Ctrl/Cmd is down when selection box is released, nodes will be deselected instead.
Deselect all nodes by clicking on the background with Ctrl/Cmd down.
If selected node is moved, all selected nodes will move with it.

examples/demo.rs

@@ -175,7 +175,7 @@ impl SnarlViewer<DemoNode> for DemoViewer {
                 match &*pin.remotes {
                     [] => {
                         ui.label("None");
-                        PinInfo::circle().with_fill(UNTYPED_COLOR)
+                        PinInfo::star().with_fill(UNTYPED_COLOR)
                     }
                     [remote] => match snarl[remote.node] {
                         DemoNode::Sink => unreachable!("Sink node has no outputs"),