ttools: minor tweak which may improve memory usage

Reorder data cache manipulations so that the old ones become available
for discarding before the new ones are acquired.  I'm not sure if the GC
is smart enough to spot this however.
topcat: add progress bar to plots for data loading

Implemented a new DataStoreFactory class, which wraps an existing one
but decorates any RowSequences used by the associated tables to update
a given JProgressBar model.