Gantt chart improvements

[generoso]

NOTE: This is a high level issue, that must be split in sub-issues after

Create a new Gantt plugin improving the existing one, trying to achieve the following goals:

• use JavaFX and Java 8
• provide a vertical zoom
• improve the state aggregation algorithm using a Paraver-like behavior; i.e., use the dominant state for horizontal (temporal) aggregation.

You can take inspiration from Aftermath for the Gantt aggregation algorithm for the horizontal zoom.