topcat: refactor GangLayerControl

Functionality is now split between abstract class FormLayerControl
and concrete subclass MultiFormLayerControl.  This will allow a
separate concrete subclass SingleFormLayerControl to use the same
superclass code.
topcat: accommodate Histogram properly in plane plot GUI
ttools: add bar shape config options for histogram

Add config options BarForm, line thickness and line dash.
Non-default bar forms also require some additional work to plot with
a flipped Y axis, since the classic plot BarForm class relies on the
bars pointing upwards.
ttools: improve options for histogram bin width sizing

Use round numbers for bin widths, and allow explicit setting of fixed
bin width sizes.