ttools: note and work around error in PlaneAxisAnnotation.

The getNoScrollTickPadding method seems to be broken, so for now
just use the getScrollTickPadding method instead, which will give
a reasonable result.  Fix it one day.
ttools: add insets parameter to Plot2Task

You can now specify how much space you want outside the data area of
a plot when doing a stilts plot2 plot.  This is useful if you want
detailed control over where the plots will appear on a page.

Implementation involved some refactoring, providing more public access to
processing done by PlotPlacement static methods and moving some code
out of PlotDisplay and into Plot2Task.
in PlotPlacement and