factor out icon asset

in the process of moving to fully customized builds, we should define the app icon asset at compile time, and get rid of the .svg living on this repo (used by notificator.go). we also could define a default asset and override it optionally in vendored builds (although this makes less sense to me).

another option would be to try to get a predefined asset name by a path relative to the binary. @meskio: which option does sound better for you?