Make the axes uses the whole plotting area

[Juan Pablo Caram]

Maintain scale ratio. Determine aspect ratio of geometry, set the range of the larger axis. Expand the other axis range to cover the current dimension of the container.

[Juan Pablo Caram]

App.zoom_fit() has been modified to adjust the plot limits such as to have the same aspect ratio as the canvas. The aspect ratio of the axes will not automatically adjust after the canvas is resized (resizing the window or panes for example). Responding to such event would be an option, but with the current implementation it would be too slow. This will be postponed until performance is optimized.

[Juan Pablo Caram]

Responding to "configure-event" on the canvas the axes are now adjusted dynamically depending on the current size/aspect of the canvas. Done similarly as was done with zoom_fit, and not the common parts are in adjust_axes.