Make zoom level and time scale uncorrelated

[lfasani]

Currently, when the zoom level is changed using Ctrl+mousewheel the time scale displayed in the upper part of the gantt is changed consequently.

Feature:
We should be able to choose independently, the time scale displayed and the zoom level
If the time scale is very thin (hour for example) and the zoom very large (we can see a full year for example), the display of the time scale would be completly overloaded. So some to small time scale should be forbidden depending of the zoom level.