icons: Install Quassel-specific icons in hicolor for Qt4

Qt4 sadly doesn't support split icon themes yet, so our injection
and mechanism does not work.
Install the Quassel-specific icons into hicolor, so they should be
found at least in a proper system installation. Use the Oxygen
theme, because its layout matches the hicolor one (simplifying
the install rule), and it was default for Qt4 anyway.