moved bitcoin.svg to correct folder (qt/res/src)

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
new windows and linux icons

- new windows .ico contains multiple resolutions up to 256px
- new testnet (green) icon
- new png icon for llinux, etc.
- new doxygen icon
- changed the assets-attribution.txt

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
new icons: overhaul oxygen icon (was to blurry)

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge pull request #2477 from jonasschnelli/new_icons

new icons for windows, linux, etc.