overhaul of the testnet icon (green color) and the toolbar icons

- redefined the green color
- created new toolbar icons
- updated the assets-attribution.txt

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
better quality windows .ICO icon file

- icon quality of the win icon of <256px resolutions are now much better

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge remote-tracking branch 'jonasschnelli/new_icons' (#2497)

Conflicts:
	doc/assets-attribution.txt