whitespace cleaning
libpng: keep versioning in all-packages.nix, not in individual expressions
libpng: merge apng expression (keep patch disabled by default)

I made sure no rebuild will happen because of this.
Unfortunately this meant merging attrsets instead of just using an
optionalString for postPatch
kde 4.10: option to use a libcanberra with gtk support

by default (as it was before), kde depends on a libcanberra with its
gtk dependency removed so a kde system can be gtk-free.

However, both firefox and chromium depend on gtk+ and libcanberra too.
So this leads to 2 versions being installed. this option combats that
by letting the kde use the other libcanberra
ffmpeg-1.x: passthrough vdpauSupport
firefox 22: use system libpng
eclipse: use latest cairo/gtk

Eclipse seems to work fine without this now (after x-updates merge).