IGV: Improve Zooming

1) Add a minimum and maximum zoom level. If you have a sensitive mouse wheel, it can be annoying to zoom in or out too much (until the graph is invisibly small or the nodes are larger than the window)

2) Zooming with a trackpad is not very smooth for two reasons:
    a) it jumps between zoom levels 100% to 130% and back to 100% again - make this more smoothly.
    b) IGV does panning and zooming at the same time - disable panning when CMD/Ctrl key is pressed for zooming

3) When only a few nodes are selected, zooming is no longer mouse centered. Instead, the center of the zooming is in the upper left corner. Make this mouse centered as well.

4) Add a button / shortcut (e.g. Ctrl - 0) to reset the zoom level to 100%.