Fix messagelist core includes.
There is a test in Kubuntu's packaging which builds the installed library headers alone, detecting problems such as having other headers #include'd which can't be found.
Right now, without this patch, the compilation of 'widgetbase.h' would fail because it won't be able to find '#include <core/enums.h>', so I replaced this with '#include <messagelist/enums.h>' and I also applied the same change to other headers which otherwise wouldn't be found.
It would be nice to get this problem sorted out in KDE's code, so this way we won't have to patch it in the packaging.