Use kmail-symbolic icon for system tray

Icons are being removed from Plasma themes and imported into breeze. To avoid having the panel icon appear as the app icon it was renamed to kmail-symbolic in breeze icons. Because of the icon theme lookup rules, icon themes without a "kmail-symbolic" will use a "kmail" icon. When running on an older Plasma with a "kmail" override this will also be used instead of "kmail-symbolic".

See frameworks/breeze-icons@7bff0845 and frameworks/breeze-icons@baaf42da