This is a simple change to allow user to launch Skanlite's scanner preview with D-Bus global hotkey (via KHotkeys app in particular).
This functionality depends on KSane patch: https://phabricator.kde.org/D16795
Note: the new slot is connected in "modern" way which checks slot's existence at compile time and throws an error if it's not found. It may be changed to old-fashion one to make sure new Skanlite builds without error with old libKF5Sane.so (the error message will be printed to stderr at runtime in this case).