Fix login and logout, minor UI improvements

The following commits fixes few bugs:
-Unable to log in (the object FXPUtility was in disposed state on creation because UserId was 0)
-Crash when trying to log in after unsuccessful login
-Crash on exit when user logged in multiple times
-Crash on exit after unsuccessful login
-No feedback when logging out

Other changes:
-Password characters are hidden now
-Only one login is allowed at a time
-Hit enter to log in when typing username or password