Smooth zoom desktop

[dgarciabriseno]

For #519

This implements smooth zooming in/out for desktop browsers.

New Updates:

• The zoom in/out buttons now animate zooming in and out.
• Scrolling the mouse wheel now performs the same actions as pinching does on mobile devices.
• event markers, info dialogs, and trajectories now maintain a constant size while zooming in/out (previously the markers would also grow/shrink).

Fixes:

• Earth scale position in screenshots/movies on mobile devices is fixed. Previously the computation for the position of the scale box was wrong after smooth zooming in or out (This only affected mobile devices).
• Computing the viewport region of interest now takes the smooth zoomed image scale into account by default.
• Celestial body labels appearing behind the trajectory lines when it should be on top.