Add support for hidden time frames in charts

[PierreRaybaut]

Currently, PyPlanning is showing Gantt tasks in charts according to their respective duration.
When a project has simultaneously short-term tasks (over weeks) and long-term tasks (e.g. a 1-year warranty), the whole chart may be hardly readable and the user is forced to truncate the long tasks if they are located at the end of the project (or to do other image cropping and manipulations if they are right in the middle of it).

The purpose of this enhancement is to add support for hidden time frames in charts.

Tasks:

• Modify the Gantt visualization module to be able to mask certain periods of time: this is the hard part, and may jeopardize this enhancement proposal because it is really not aligned with the current drawing mechanisms (tasks are drawn one by one)
• Add the hidden periods to the data model (both XML and internal data model)
• Add the possibility to enter the hidden periods from the GUI