libs/Common: changed DatabaseInterface to use SLOT(method(...)) instead of "method"
apps/Sorcerer: changes to match SLOT(...) changes
apps/Karmadrome: changes to match SLOT(...) changes
apps/Rubberbandman: changes to match SLOT(...) changes
tests/TestAppCoverArt: several updates
apps/Rubberbandman: added missing default argument
libs/Common: added function to access QImage of ImageWidget