Several usability improvements for dotty:
1) Center the node's bounding box on the nodes position (in the global coordinate system) based on it's size while being moved. This solves the issue of being unable to click-and-drag a node after it was initially dragged, since the bounding box erroneously remained at the initial position. 2) Keep the relative position offset between the center of the node and the position of the cursor when clicking-and-dragging. This avoid snapping the center of the node to the current cursor position, but rather allows it to move with the cursor. 3) Redraw the graph on 'leftup' to correct nodes that may have become visually corrupted by edges obscuring their outlines.
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