Improved logging system

Right now the logfile is a pretty mess (IMHO) as it is easily flooded as it collects every message and output. Furthermore it is hard to connect an entry to a desklet.

Improvement ideas:
- every running desklet will have its own logfile in ~/.gdesklets/logs/
- the logfile will use the desklets ID to be identified (name: ID.log)
- the logfiles will be deleted with every gDesklets startup or if the desklets is restarted (and/or stopped ?!)
- the about window could be extended with a new "Logfile" button which will open a new logview window including a "copy log to clipboard" button to make it easier for the user to send in error reports
- convert gDesklets to use the python logging module (e.g. logging.info(), .error(), .debug(), etc. instead of the print statements)