Refactor mouse event handling in graphs

[rensink]

I've refactored some of the mouse handling routines for the graph editor. Most noticeable changes are:
* Minimum drag distance added for usability.
* Callback for popup menu, popup menu no longer displayed after right-click drag operation or scroll.
* Moved selection handling to press/release handling rather than clicked.
* Edge creation by dragging with right mouse button. Although not entirely intuitive according to standard GUIs, I feel this might be less obtrusive than the current single click behavior.
* Dragging/zooming the graph while in selection/edit mode (no explicit switch to pan/zoom mode required):
- Panning with dragging middle mouse button.
- Zooming with scrolling middle mouse button using ctrl or right mouse button depressed. The position that is zoomed into remains fixed at the cursor.

The autoscroll is still interfering with some drag operations (for example, it tends to make the panning look jerky). This might need to be fixed at some point too, but it does not affect the new implementation any different than the old one.

Reported by: harrybharry

[rensink]

New JGraphUI file

Original comment by: harrybharry

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I've updated the patch somewhat to fix a few bugs and to make the autoscroll (in my opinion) more useful.

Original comment by: harrybharry

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Patch for svn rev 4438

Original comment by: harrybharry

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• Group: --> None

Original comment by: rensink