This commit introduces a skeleton that allows personalized callbacks in
a backwards compatible way. Settings 'on_lclick','on_rclick',
'on_scrollup','on_scrolldown' are inherited by all modules.
These parameters should be a string. Then when a matching action is
detected (ie mouseclick, scrolling), the module check if this string
corresponds to a:
1/ python callable
2/ module method,
In cases 1 and 2, it calls the python function with the module as the
first parameter. Otherwise it considers the string is an external command and launches it via run_through_shell