Improve access to debug logging

[cefect]

For debugging and resolving errors, the user often needs to access the debug level messages which are only sent to the QGIS_LOG_FILE , the location of which might be unknown to the user and difficult to find. To improve user experience, I propose the following:

1. automatically configure QGIS_LOG_FILE if it has not been set yet when the plugin is first loaded
2. include a small 'open log file' button on the CanFlood menu drop down to launch the log file with the system default text editor (QgsLogger.logFile())

[NickyHastings]

good suggestion