More ways of assigning IDs to label when exporting label image

[tinevez]

Before this PR the labels in the label image had either the spot's track ID (+1) or the spot's ID (+1). This was unsatisfactory for several reasons (see https://forum.image.sc/t/trackmate-spotsid-initial-value/76753/5)

This commit adds two ways of assigned IDs to labels:

• index starting from the 1, unique in a frame.
• the same, but unique in the movie.