utils: update the package API

- Remove the package singleton and provide a public constructor.
- Make the distinction of the source and binary package in the API using
  the enum type (PKG_BINARY, PKG_SOURCE).
- Add a package instance to GVLE.
- Add the argument curr_package to gvle modelling plugins.
- Output files are now generated into the current dir for vle and rvle
  and into <current_package>/output into gvle.
gvle: update the files management

- The dialog used to start a new project has been removed. Now we use a
regular file dialog. It does enable to choose a directory anywhere, to
be the recipient of a package.
- The Global VPZ features have been removed.
- The regular file chooser dialog is used to open a vpz file, and the
older vpz chooser dialog has been removed.
(closes #82)