Introduce Spell Check settings configuration view in digiKam Setup/Miscs.

The view only host the single option to turn on/off the spell-checking in background.
All text edit widget supported spell-checking are affected by this change without to re-start application.
More options/info about spell-checking will be hosted in this view.
This view will be also appended to Showfoto/Setup later to be homogeneous.
CCBUGS: 376345