Create config class, to handle session and parameters to filesystem.

[ojob]

This is linked to issue #38: remember last visited directory.

Access to configuration data can be performed by attribute or as dict entries.

File is in YAML format, and is written automatically at exit.

I choose to make interface with other Gtk classes 'simpler', i.e. at a lower level: these do not access Options instance directly, but directly to parameters or to parent instance.

Signed-off-by: Joël Bourgault [email protected]

[ojob]

This is a rebase of previous work on merge request #52, in order to get a cleaner history.