Make it possible to load from a partial TrackMate XML.

For instance, MaMuT files miss a detector & tracker element, and their
GUIState is not set.
This commit makes it possible to load such a file and display the model
in a default blank view.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Fix several issues with the zoom in TrackScheme.

- Fix the zoom issue with the simple style: when zooming out using this
  style, the scrolbars would disappear and confuse the graphComponent.
  This was caused apparently by choosing mxPerimeter.EllipsePerimeter for
  this vertex style. Replacing it with RecntaglePerimeter fixes the
  issue and conserves the look of TrackScheme.

- Make the zoom more responsive. Do not display the graphComponent
  before translating to the area of interest.

- Keep selection in view when zooming.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>