Usability improvements

- Changes menu item 'Change shape palette' from menu 'Edit'
to menu 'Palette' and adds a confirm dialog to this
dangerous operation.
- Adds buttons 'Add new rule' and
'Create new Interaction Net rule'.
- Removes buttons 'Add new agent' from dialog
'Create new Interaction net rule'.
- Test if palette without interface agent is empty in
'Create new Interaction net rule'.
- Readjustment in ports's colors; port of interface agents
are painted differently.
- New graphical way to generate textual descriptions
- Possibility to generate partial textual descriptions;
just rules or just net.
- When loading new palettes, checks if the set of new
names contains the set of old names.
- Loading the palette is not a document operation anymore.