Update Quassel-Icons based on justjannes icon-repo

See https://github.com/justjanne/quassel-icons commit 18a8974
inkl. new slim Tray-Icons
inkl. Inverted Tray Icons (Light to Dark and vice versa)
Updated Quassel App Icon
Put Extra Icons in a separate folder in Resources/Data
Updated quassel.ico and .icns