Make mailfilter's dummykernel less dummy, and also provide a Settings interface to it.

This properly fixes 286762 (the previous fix just postponed the crash to a later stage).
Remove MailCommon::Kernel::settingsAreRegistered, as it doesn't make sense to have it (SettingsIf is mandatory).

BUG: 286762
CCMAIL: kde-pim@kde.org