Add preference keys for 'Restrict allowed versions' & hide 'Support Xcodes'

[abiligiri]

• Introduced a new preference keys allowedMajorVersions, hideSupportXcodes
• allowedMajorVersions defaults to Int.max (ie allow all versions till date)
• allowedMajorVersions is used to limit the number of major versions to as many as value set for this key. Eg: A value of 1 would allow the latest GA version and one major version before A value of 0 would allow only the latest GA version A value of 2 would allow the latest GA and previous two major versions
• allowedMajorVersions does not have preference UI $ defaults write com.xcodesorg.xcodesapp allowedMajorVersions 2 #to limit to current GA and previous major $ defaults delete com.xcodesorg.xcodesapp allowedMajorVersions #to remove limits
• Display buildNumber in bottom status bar