Show traces not working with slits created on the fly

[mrobberto]

Having reinstated the draw slit mode, it turns out that the draw trace does not work on these slits; the reason is that the draw traces work on slits with tag=loaded, that is assigned to the slits taken from the region files, but not to those created on the fly.

[koeppeda]

I created a branch for this issue and will set aside time to look into it.