ttools: abstract mouse button number handling

All determination of which mouse button is in use (changed or pressed)
now goes via the PlotUtil.getButton*Index methods rather than examining
the MouseEvent directly.  The point of this is to have control over
what mouse/keyboard gestures count as logical button 1/2/3.
It's not straightforward to work out what should mean what across
different OSes and mouse devices, doing it like this means we actually
know what's going on, and can document it at the user level accordingly.
ttools: refactor captioner configuration

The logic for selecting a font from configuration options is now
broken out into a new class CaptionerKeySet rather than using
opaque logic within the StyleKeys class.  This means it can be
reused, which in turn means the LabelPlotter class is able to use
a different instance of the relevant config keys than that used
for the axis labelling.  So now the logic in the topcat GUI classes
lets you choose a different font for point labelling than for
axis labelling.
topcat: prevent autopopulation of non-positional coordinates

This fixes a bug which was an oversight at 030b0484.
topcat: withdraw Labels tab from sphere plot window Axes control

It didn't work anyway.  Should ideally be reinstated and fixed at some
point in the future, but probably won't be much used.
ttools: fix logic in X/Y axis anchor config options

They were the wrong way round, since I was confusing X Axis with X=0 line.
topcat: first go at reorganising change list for v4.1.
topcat: rearrange sun253 sections on new/old-style graphics windows
topcat: update sun253 text and figures for 4.1
topcat: add new navigation text to sun253
topcat: add a help button to the NavigationHelpPanel

Add a little button in the footer panel to bring up navigation help
specific to the current plot2 window.
topcat: version to 4.1 for release.

stilts to version 2.5-2.
topcat: add section to sun253 on plotting xmatch results