vo: use RegTap ivo_string_agg function to query subjects better

String aggregate function added to the RegTAP standard makes it
possible to write more efficient and less baroque registry queries.
vo: RegistryPanel now displays resource count

A Resource Count indicator is displayed at the bottom of RegistryPanel
showing how many resources are visible in the current list.
util: move Downloader from topcat to util.gui
vo: add progress monitor for registry endpoint updater

When the list of registries is queried to update the list of available
registries, Downloader monitor component is displayed to show progress.

This is useful, and an improvement, but it should really work
differently than this - there ought to be a single registry selection
model for the whole application not one for each window.  One day.
topcat: document new registry GUI

Update the user document to explain the registry protocol selector,
with some short notes on the difference between RegTAP and RI1.0.