Use ecm_qt_declare_logging_category to declare the logging categories

This automatizes the generation of logging categories so a kdeconnect-kde.categories file is generated and installed to /usr/share/qlogging-categories5/ so kdebugsettings can use it.

Also, sets the default logging level to Warning. So now the logs of users won't be filled with debug messages but they can modify the configuration easily with kdebugsettings.