Filtered BED tracks not saved to IGV / UCSC sessions

[dievsky]

Steps to reproduce:

• open any scored BED track and apply a filter or tune it
• save the session in IGV or UCSC format

Expected result:

• the track is saved to the session as an unfiltered BED, or
• a warning is displayed that a filtered track can't be saved to IGV / UCSC

Actual result: the track is silently dropped from the saved session.