Use a RAII class instead of blockSignals(true);...;blockSignals(false)

A nasty "return" statement in the middle of favoriteSelectionChanged()
was leaving signals blocked.

KDSignalBlocker is from KDTools (LGPL), modified to be in the Akonadi
namespace (to prevent clashes with the copy in kleopatra), but not exported
currently -- could be exported+installed if needed.