Time unit management

[generoso]

• [ ] Add a new trace metadata corresponding to the displayed time unit. This time unit shall be used in all visualization (timebar, gantt timestamps, ..) and also in statistics computation related to time (state duration, link duration).
• [ ] The displayed time unit shall be modifiable by the user, for example using a combo box where she can select the unit she wants (this is done in the metadata view). Note that the other time unit is still unchangeable, since it corresponds to the raw trace data.
• [ ] The displayed time unit should be shown on time axis. This makes the view self explaining: the user does not need to check the metadata view.
• [ ] The time format (e.g., "###.#E0") should be defined globally in the configuration file. It should be possible to change this format for a specific view.

[generoso]

Finally a different, cleaner and simpler solution has been implemented. See issue #87.