Dynamic contextual menus

Hi,

The current contextual menu is far from perfect at the moment. There
should be no "Save to file" entry for charts and for all embedded
objects when the plugin is not loaded. In the latter case, the "Format
Object" entry is not useful either. BTW, I'd prefer "Edit object" which
seems more intuitive.

To fix these problems, I worked on a dynamic contextual menus mechanism.
The attached patch contins the relevant changes. I have not implemented
the XAP_Menu_Factory::removeContextMenu method yet. At the moment things
seem to work without it.

The getContextualMenu method has been implemented in FP_EmbedRun, but it
might be generalized to FP_Run as a virtual method.

Cheers,
Jean