logger: Refactor the logging framework

* Rename the previous Logger class to LogMessage (since that is what
  it is), and consolidate logging functionality, including the
  various output methods, setup of log file etc., into a new
  Logger class.

* Hold the "debug log", i.e. the collected log messages, in the Logger
  class instead of in the Client. Adapt the DebugLogWidget accordingly.
  Messages will not be kept in quasselcore, as we wouldn't have a way
  to display them otherwise (and we don't want to accumulate lots of
  data over months of the core running).

* Support early log messages. Instantiate the Logger right at the
  start of main(), and keep all log messages until the Quassel class
  is initialized and we can rely on command-line options to configure
  backends accordingly. Accumulated messages will be output once the
  Logger is configured.