ebuild: remove bash-4 specific code

fixes #388
optimize graphics using matthiaskrgr/image_optim

stats: 148 binary files changed 278933 b (272 kb) -> 159998 b (156 kb) => [-118935 b (-116 kb)]
Merge pull request #394 from karolherbst/ebuild

ebuild: remove bash-4 specific code
Merge pull request #407 from matthiaskrgr/optimize_graphics

optimize graphics using matthiaskrgr/image_optim

stuff done by this commit:
1. conversion of some files to grayscale pngs + drop of PLTE table
2. removement of iTXt and eTXt tags created by adobe software
3. removement of "created with photoshop" tags

the image raw data does not seems to be touched.