topcat: adjust plot interruption logic

When a UI action triggers a new plot, it's hard to work out whether to
cancel one that is currently in progress.  I noticed a regression issue;
the current implementation is less responsive when dragging sliders to
change the density scales etc.  Try to fix this up (success) without
making other things work less well (?).
It's hard to work out whether to cancel a previous plot when starting a
new one (triggered by a UI action).
ttools: add hooks for specifying pixel compositor

The Compositor is no longer hard coded into the PaperTypeSelector,
clients of that class must supply their own.  Plot2Task provides a
parameter for specifying this (saturation or a specified boost).
Currently topcat does not, it still uses a hardwired value,
but the hook is there for changing it in the future if required.
topcat: fix shader control so it doesn't disappear too much

The shader control is only visible in the stack if one of the plotters
in the stack is capable of using it (producing layers with aux shading).
However, if this is interpreted too literally it can disappear in an
annoying way, e.g. if you are just hiding one of the layers.
Fix it so that the shader control doesn't disappear unless there are
zero aux-capable layer controls in the stack, rather than just zeor
active aux-capable layer controls in the stack.
topcat: re-use selected coords when new table selected in plot

When a new table is selected in a plotting CoordPanel, try to re-use
the strings entered into the column selectors.  This will only work
if the columns in the new table are sufficiently similar to the old
ones, but where it does work it's probably what's wanted.
ttools: provide slider as alternative to spinner for IntegerConfigKey
ttools: add crowding config options to LabelPlotter

Allow to control how many labels are plotted within a few pixels of
each other.  This has two components: spacing to define the distance
within which other labels are allowed, and crowdlimit to define the
maximum number of labels in that region.