Migrate from KSystemTrayIcon to KStatusNotifierItem

 - Use KIcon instead of QPixmap
 - Deprecate showPart, seems not to be used, produces a No such signal msg
 - Replace quitSelected with the signal of KStatusNotifierItem's default quit action
 - delInitTrayIcon() -> setTrayIcon()
 - improve TrayIcon::slotSetUnread() based on KMSystemTray::updateCount()

Thanks to Theofilos Intzoglou <int.teo@gmail.com> for his help

REVIEW:100850