Turn the "-split" option in bedtools plugin into a checkbox

Remove "-bed" option from arguments, just output it always
Change data track math input to a dialog, accessible from tools menu

IGV-1931
Some work on saving/loading CombinedDataSource track into sessions

Not done yet

IGV-1931
Document writing igvtools output to standard output

IGV-1945
Allow '#' to be used as a comment in $$_dataRegistry.txt file

Skip blank lines instead of breaking on them
Properly handling saving/loading of renderers into session files for DataTracks

IGV-1931
Remove documentation of stdout command

Fix bugs in writing to stdout from IGVTools, were getting extraneous log statements

IGV-1945