topcat: report OutOfMemoryErrors visually during plots
ttools: add padding to auto-ranged plot limits
ttools: note a small colour discrepancy in RgbImage

This discrepancy is not significant in terms of the output graphics,
but it may indicate a performance issue.  Mark it for possible later
examination.
ttools: add transparency config item for histogram layer
ttools: add grid and axis label colour options for plane plot
ttools: add grid and axis label colour options for sky plot
ttools: add grid flag status to TimeSurface hashCode/equals methods
topcat: add navigation help panel

Navigators now report the mouse gestures which influence them.
Topcat in turn displays this information in a panel at the bottom of
the plot window.
topcat: remove point selection functions for histograms

For some plot types (currently only histogram) things like blob selection
and clicking to identify a point are never used.  Mark these plot types
in the PlotTypeGui interface and use that marker to remove those options
from the GUI where appropriate.