TrackMate_: manual editing of links in the HyperStack view

A new function can be accessed in the base viewer for TrackMate:
pressing the 'L' key when two spots are in the selection will create a
link between these two spots, or remove one if there is one.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
TrackMate_: clicking outside any spot clear current selection

Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
TrackMate_: fix a potential bug in spot IDs when loading a model

We use a counter to generate spot IDs. In some case, it could be
possible that the ID counter was not properly initiated and could have
generate new spots with an ID already assigned. Brr...
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
TrackMate_: auto-linking mode in HyperStack view

Press shift-L to toggle this mode. The newly created spot will be
automatically linked to the one currently selected.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>