Added icons for system theme & userChrome.css for integration

Слиты Варфоломеев Алексей Александрович запросил слияние icons в main

userChrome.css need build to omni/chrome/classic/skin/classic/messenger/shared/userChrome.css and added import @import url("chrome://messenger/skin/shared/userChrome.css"); on file: mostechmail/comm/mail/themes/shared/mail/about3Pane.css