Store the zoom factor as an integer number

For now, TimePaneView.zoom_factor is a float number. At each zoom in (out) step, it is multiplied (divided) by 1.2 (the zoom step). Due to floating point precision problems, the zoom factor may not come back to the value 1.0. The “zoom factor” should be stored as an integer number that is incremented/decremented when the scene is zoomed in/out. The multiplicative factor would be computed as zoom_step ** zoom_factor / zoom_step ** (1 / zoom_factor).