Enhance the destination control parameter of wfDebug() and wfDebugLog()

- The parameter is now a string, making is more understandable than
  boolean values
- It takes the same values in both wfDebug() and wfDebugLog() (except
  for 'private' which is only used in the latter)
- This adds a new possibility to wfDebugLog() to log the message either
  on the specific log or the general one, but not to the debug toolbar
- Old boolean values are still recognised for backward compatibility
- Also send the messages passed to wfDebugLog() to the debug toolbar
  when they are written to a specific log and not restricted to logs
- Updated the calls of and wfDebug() and wfDebugLog() with the last
  parameter to change it into a string
- Renamed MWDebug::sendWarning() to MWDebug::sendMessage() and added
  $group parameter to it; will not break anything since that method
  is marked as private
- Changed the call to wfDebug() from MWDebug::sendMessage() to use
  wfDebugLog() with 'log' as thrid parameter, so that those messages
  can be logged separately from the main log and they don't show up
  a second time on the "debug log" tab of the debug toolbar

Change-Id: I1be09d4c1d3408ed5b26a5db02691c17c0ec0926