Change to Marble logging behaviour and command line

[Jonathan Marten]

Since the merge of
https://invent.kde.org/education/marble/-/merge_requests/114 and
https://invent.kde.org/education/marble/-/merge_requests/118, Marble now uses 
the standard Qt logging system for debug messages.  This should not affect 
users of the application or library, but there some changes that developers 
may need to take account of:

- Whether debugging messages are shown, and the format used to display them, 
are controlled by the standard Qt logging settings.  See the descriptions of 
QT_LOGGING_RULES and QT_MESSAGE_PATTERN in
https://community.kde.org/Guidelines_and_HOWTOs/Debugging/Using_Error_Messages
for how to configure debug output.

- In Plasma the debugging settings can also be controlled by the 
'kdebugsettings' utility.

- For the Marble desktop applications 'marble' (KDE) and 'marble-qt', the
'--debug-info' command line argument is deprecated and does nothing.  It will 
be accepted, so as not to give an error if used in a script or desktop file, 
but is ignored and will show a warning message.

- The library functions MarbleDebug::setEnabled() and MarbleDebug::isEnabled() 
are again deprecated.  setEnabled() gives a warning message, accepts and sets 
the option but does nothing.

- Logging within Marble code may continue to use mDebug(), which will use the 
logging category defined by the LOGGING_CATEGORY compiler definition. 
Currently three logging categories are defined in
src/lib/marble/MarbleDebug.cpp: MARBLE_DEFAULT, MARBLE_LIB and MARBLE_PLUGINS.  
New code may use qCDebug() with the appropriate category.
                                                                                                                                                                                                                                                                                                    
Please of course raise a bug for Marble (at bugs.kde.org) if this appears to 
be not working or if are any problems.

Best regards,
  Jonathan
-- 
Jonathan Marten                         http://www.keelhaul.me.uk
Twickenham, UK                          jjm2 at keelhaul.me.uk