Add GLib logging support with python wrapper functions

Wrap g_log() from GLib (unfortunately it is not provided by PyGTK) and
add a convenience wrapper in Python to behave like a
logging.Logger object.
Additionally, in geany.Plugin this wrapper is instantiated on Plugin
initialization and LOG_DOMAIN is set accordingly.
This enables GeanyPy plugins to log through GLib's logging
system like any other plugins and Geany itself do and so log
messages will be visible in Help->Debug Messages.