gimp: change default from 2.6 to 2.8
gimpPlugins.ufraw: update plugin from 0.15 to 0.19.2 to build with gimp 2.8
gimpPlugins.texturize: fix compilation with gimp 2.8
gimpPlugins.fourier: upgrade from 0.3.3 to 0.4.1 and fix compilation
gimpPlugins.gmic: upgrade from 1.3.2.0 to 1.5.7.4, fix compilation with gimp 2.8

Only the gimp plugin is built now, as the rest of the suite is quite
large and probably deserves its own package.