Rebuild Scala projects after a global compiler settings change.

If the user changes compiler settings on the Scala Preferences page, all Scala
projects that 'inherit' these settings should be rebuild. The existing logic
was dealing only with the case where *project-specific* settings were changed.

Fixed #1001460.

Note: there is no test since this action is triggered from the 'ok' handler of the preference page. I think the changes are quite well-localized, so that shouldn't be a problem.