In GimpHistogram, get rid of the "n-channels" property and
corresponding gimp_histogram_n_channels() function.  The former
returned the actual number of channels, but this wasn't too useful,
as channel values may not be sequential; the latter returned the
number of components.  Instead, add an "n-components" property and
a corresponding gimp_histogram_n_components() function, both of
which return the number of components.  Furthermore, add a
gimp_histogram_has_channel() function, which determines if the
histogram has a given channel; this allows for simple testing for
channel availability, which was done wrong in various places.

Adjust the GimpHistogram code for the changes, and clean it up,
fixing a few bugs.

Adjust users of GimpHisotgram for the changes.  In particular,
in GimpHisotgramView, fix the channel-availability test when
setting the view's histogram (which happens whenever the active
drawable's preview is frozen), ...