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Integrate channel mixer into full program

Menu icon, etc.  The Color menu layout was also modified slightly to
keep things clean.

In this commit, I have also modified the channel mixer to allow for
mixing all channels at once.  This is the behavior in Photoshop/GIMP and
it allows for pretty complex operations.

Monochrome has also been modified slightly to match the aforementioned
programs.  The monochrome checkbox now disables the channel combo, and I
have removed "gray" as a channel option.  I think this provides the most
understandable behavior.

Unfortunately, the complexity of modifying all channels simultaneously
means optimizations like look-up tables can no longer be used.  But
because the math involved is all simple multiplication I don't foresee
any problems.  Performance remains pleasantly good.