Use QMobipocket6 as name for the Qt6 build variant, to allow co-installation

Also install headers of Qt5 version into a visibility prefix QMobipocket/. Prevents headers to be seen when not linking to this version of the library.

To give a better idea, here what is deployed now:

Built against Qt5 (only difference to before: additional QMobipocket/ with includes)

-- Installing: <prefix>/include/QMobipocket/qmobipocket/mobipocket.h
-- Installing: <prefix>/include/QMobipocket/qmobipocket/qfilestream.h
-- Installing: <prefix>/include/QMobipocket/qmobipocket/qmobipocket_export.h
-- Installing: <prefix>/lib64/libqmobipocket.so.2.0.0
-- Installing: <prefix>/lib64/libqmobipocket.so.2
-- Set runtime path of "<prefix>/lib64/libqmobipocket.so.2.0.0" to "<prefix>/lib64"
-- Installing: <prefix>/lib64/libqmobipocket.so
-- Installing: <prefix>/lib64/cmake/QMobipocket/QMobipocketTargets.cmake
-- Installing: <prefix>/lib64/cmake/QMobipocket/QMobipocketTargets-debug.cmake
-- Installing: <prefix>/lib64/cmake/QMobipocket/QMobipocketConfig.cmake
-- Installing: <prefix>/lib64/cmake/QMobipocket/QMobipocketConfigVersion.cmake

Built against Qt6:

-- Installing: <prefix>/include/QMobipocket6/qmobipocket/mobipocket.h
-- Installing: <prefix>/include/QMobipocket6/qmobipocket/qfilestream.h
-- Installing: <prefix>/include/QMobipocket6/qmobipocket/qmobipocket_export.h
-- Installing: <prefix>/lib64/libQMobipocket6.so.1.0.0
-- Installing: <prefix>/lib64/libQMobipocket6.so.1
-- Set runtime path of "<prefix>/lib64/libQMobipocket6.so.1.0.0" to "<prefix>/"
-- Installing: <prefix>/lib64/libQMobipocket6.so
-- Installing: <prefix>/lib64/cmake/QMobipocket6/QMobipocket6Targets.cmake
-- Installing: <prefix>/lib64/cmake/QMobipocket6/QMobipocket6Targets-debug.cmake
-- Installing: <prefix>/lib64/cmake/QMobipocket6/QMobipocket6Config.cmake
-- Installing: <prefix>/lib64/cmake/QMobipocket6/QMobipocket6ConfigVersion.cmake

Usage would be like this

With Qt5 as before:

find_package(QMobipocket 2.0)
target_link_libraries(foo qmobipocket)

With Qt6 (see also frameworks/kfilemetadata!80 (merged)):

find_package(QMobipocket6 1.0)
target_link_libraries(foo QMobipocket6)