Fiber selection and display behavior

Fixed two bugs related to Fibers.

The first one caused the Fibernavigator to crash when perfoming the following steps:
Convert more than one RTTFibers onto a Fiber dataset, due to the name used as id for fiber selection.
Now using a specific id.

The second consists in a wrong display of selected fibers, mostly occuring when enabling/disabling their Show state.
Fixed it by raising a flag so that each time a Fiber dataset (or FiberGroup) is re-enabled (onShow == true). This way, SelectionBoxes are updated.

Fixes #138, #139