ECMQtDeclareLoggingCategory: support kdebugsettings files w/ . in basename

As needed by expectations from https://invent.kde.org/plasma/plasma-activities/-/blob/master/src/CMakeLists.txt?ref_type=heads#L43:

ecm_qt_install_logging_categories(
    EXPORT PLASMA_ACTIVITIES
    FILE kde.plasma.categories
    DESTINATION ${KDE_INSTALL_LOGGINGCATEGORIESDIR}
)

(FWIW kde.plasma.categories is not specific to plasma-activities, so people should reconsider this)

Will backport to ECM 5. Not yet tested besides reading generated docs.

@krop