WPrefs: use length of array instead of hard-coded constant for sample colours
In the panel for Appearance configuration, there are some sample colours
proposed to users; the code used to have a bunch of constants hard-coded
to handle these colours.
This patch replace all constants with the macro wlengthof to use the number
of element in the array determined by the compiler, so modifying the list
won't cause hidden bugs. This include a somehow smarter code to dispatch
the colours in the window to automatically arrange them cleanly whatever
their number is.
Took opportunity to de-CamelCase the name of the array as it did not add
complexity in the patch.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>