Update of the workind directory

[singhoff]

When a project is opened, the working directory of Cheddar must
be updated in order to reflect the directory of the opened project.

By default, the working directory is the directory from which
cheddar is launched.

This feature will avoid the user to filter its working
directory each time an AADL/XML is opened ; by the way,
it will ease the simulator engine to find .sc files attached
to XML project files.