core: replace Logger.h by Qt log functions
There should be only one logging facility, and only one log file. Different kinds of log messages shall be distiguished by prefixes. In the gui log display, we may eventually filter log lines according to their prefixes.
All log messages shall be written to one of the Qt log functions or streams defined in , qFatal, qCritical, qWarning, qInfo, qDebug. All legacy calls (Logger: debug, info, error; printf, fprintf; std::cout, std::cerr) shall be replaced.
On starting the GUI, a message handler must be set, as done in Steca/main/steca.cpp. Probably, Steca's message handler can be reused here; it should be moved from Steca/view/msg_handler to libQCR.
On starting a C++ test or a Python script, nothing needs to be done at all: If no message handler is set, then qFatal, ..., qDebug will print to stderr.