[mod0] Implement horizontal zoom

[boogheta]

The current view should be the biggest possible zoom and w should be able to use the mousewheel to enlarge/shrink the width of the time.

• in time mode, the default zoom should be the current
• in absolute mode, the default zoom should be made so that an average law covers about 2/3 of the viz
• in quanti mode I'm not sure the zoom is of any use, to be discussed

When zooming, the text within the steps should disappear if there's no space for it, and the time legend should display one out of 2/3/4/6 month names when necessary.

[fenicento]

implemented in d93df092fb270b4014d5e944f056be68cd6dd433, slow on older computers