Performance: process only Gantt objects affected by changes

[PierreRaybaut]

PyPlanning struggles to update charts when dealing with large XML files.
One way to solve this issue would be to update only the Gantt objects that are affected by changes being made in the GUI.

Here are the tasks associated to this issue:

• Rapid prototyping of the feature: the objective is to assess the feasibility of the feature (i.e. the time required to implement it vs. the gain in terms of performance)
• If the feature is feasible, do it!