Added a new GUI called 'CompareSongs' for the sake of comparing ASSL
data from different days. Right now, the programs for the color plots of
bouts and acoustic features of syllables work.
Now, feature values are also added in a convenient format, so that when
CompareSongs loads data, for each day, it gets 4 important fields,
AllLabels, AllOnsets, AllOffsets and AllFeats - these are enough for all
further analysis.