Change logos to new theme.

I changed most of the logos to the new theme, but not all of them.
More precisely, I can't find the green disc with two arrows as shown in any of the ``si-syncing-*.png`` [images](https://github.com/kozec/syncthing-gui/blob/9b9d7b02e928be50edfdd2af2e8c6896c57f642e/icons/si-syncing-0.png). Where can I find them?

This will resolve #16.
Please do not merge, yet.