topcat: fix broken screenshot

Poorly edited multi SIA window screenshot had some pixels out of place.
Paste over them.
topcat: stop doing auto-conversion of time columns

The Conversions class can automatically convert string columns to
numeric ones.  But the newer DomainMapper machinery does the same thing.
These do not play nicely with each other, and in particular make
Time plots in topcat (but not stilts) fail for ISO-8601-formatted time
coordinates.  Hack round this by turning off time conversions in
the Conversions class.  This will break some things, for instance
statistics calculations and plotting in non-time plots for
ISO-8601 columns.
However, these didn't work particularly well before (the values
were just interpreted as decimal years AD), and I'm hoping they
weren't much used.

In the long term, I think the Conversions class should be phased out,
replaced by more pervasive use of DomainMappers.
But another possibility would be to specify what kind of conversion
is required when it's used (in ColumnDataComboBox).