Zoom In/Out buttons

[rashley-iqt]

Describe the feature you are requesting

Instead of clicking a node to zoom to it use a standard set of zoom in/out buttons. This would give finer and more intuitive control over the level of zoom and free the mouse click gesture up to be used for some other feature.

[rashley-iqt]

As a side note to this, implementing this before #63 would probably both simplify #63 and improve performance. As it stands, nodes are being rendered and then a resize sensor is being applied as an overlay on the root svg element. Mouse events are being trapped at the resizesensor/root svg level and the tooltip is filling in its data by reading it from the DOM tree on mouse move. As a consequence we are rebuilding the content of the tooltip much more often than is necessary. Switching to buttons removes the need for the resize sensor and therefore allows us to handle mouse events at their correct location and type.