ttools: refactor PlotLayer implementation in ShapeMode.
ttools: add new auto transparency shape mode.

This does transparent shading but adjusts the alpha channel automatically
according to how many pixels are being hit.  The details of how it does
this could probably be improved (it currently just looks at the single
most-filled bin and scales alpha based on that).

Also renamed and relabelled the existing (fixed) transparency mode,
which the more I think about the more I think is not a lot of use
in practice, at least interactively.
task: add option to ChoiceParameter so it can take class names.

With one of the constructors, a ChoiceParameter allows a classname
as a string value, if the named class is of the appropriate type
and it has a no-arg constructor.  This is a powerful pluggability
mechanism that I should implement across the board in the TASK
package, but it relies on generics and I don't have time to do
it passim for now.
ttools: add forcebitmap parameter to Plot2Task.