Rename Kg* classes to KGame*
With the deprecated classes gone which occupied some of the names, the (newer) classes can be aligned in the naming pattern with the other API.
Needs to be merged together with respective adaptions in the games in an atomic process (same evening), but with all the games now Qt6-only and the Qt5/Qt6 change, there is a window of opportunity I would like to use. Would do all the needed work (first tests done), would go over all of them if people agree in principle.
This MR is only about the classes in the public API. Internal classes would be renamed afterwards with lower priority.
PS: Follow-up work would be to split the main lib into core, (gui) and widgets while we can break API/ABI, to prepare ground for possibly more QML work during Qt6 lifetime. Frst draft done, but would first like to have the renaming sorted.