[lmi] Fancy diagnostics for art-provider class

[Greg Chicares]

I'm trying to improve the diagnostics in the wxArtProvider-derived class
in 'icon_monger.?pp'. For instance, I recently added 16 by 16 bitmaps;
I'd like to get a warning if I overlooked one that I should have added.
If I overlooked one, I might get a scaled version of the same icon, or
one of wx's builtin bitmaps; in either case, I'd still like a warning.
But this art provider is used by wx html help, so I've exempted certain
bitmaps that are used only by the help engine.

I've modified the code accordingly on 20090112T1910Z and 20090115T1944Z,
but there's still one diagnostic:

  Image '/opt/lmi/data/help-16.png' of size 16 by 16 has been
  scale because no bitmap of requested size 16 by 15 was found.
                                                  ^^

I suppose I could add another exemption, to suppress any warning about
scaling (16,16) --> (16,15). I thought I should ask for advice first,
though:

 - I begin to wonder whether code that I had already made somewhat
   baroque is becoming downright rococo. Is there a simpler way to do
   all of this while still diagnosing the mistakes I'm likely to make?

 - I'm testing this carefully with msw only, so I could be missing an
   entire dimension. Is my recent code going to give lots of nuisance
   diagnostics with GNU/Linux? (I'd guess not, because of theming.)