this ensures that all known platforms are actually in the comobox and
reduces the amount of string duplication.
the new code gets the list of platforms from bugzilla and then either
attempts to select the one which was auto-detected by systeminformation
or recycle the one previously used.
setup is now triggered in the constructor. loading from the api is still
fairly cheap but even so takes longer than the hardcoded list, so
loading this on aboutToShow is fairly unattractive at this point.
the now useless members for combobox state tracking are gone entirely.
BUG: 383530