icons: Remove obsolete hicolor icons The hicolor directory should only contain the application icon for external use. Remove other icons (which are no longer used by Quassel anyway). Move irc-channel-joined and irc-channel-parted into the quassel- specific oxygen icon set, as they're not part of the upstream theme.
Rename the icons for inactive and alerted to conform with icon naming spec Dashes are used for level separation, so icon theme engines like Plasma's know to look into quassel.svgz if asked to provide the quassel-message icon. This fallback mechanism doesn't work when using quassel_mnessage instead, and thus breaks the rather new Air desktop theme.
Add tray icon for highlight; move tray icons to hicolor App-specific icons should be in the hicolor theme, in order to be always available. The new highlight tray icon is just a blend of our app icon and an view-pim-mail from Oxygen. This is supposed to be temporary until someone creates a better one.