Release 0.84 - Contextual menus and bug fixes

Here we are with a new sunday release. My holidays are over but I hope I'll be back next week-end with new improvements. Changes:

• Right click on view shows contextual menu (rather simple for now, more will be added layer)
• Command+Right click on view shows menu to pick layers (layers which are not opaque under the pointer are shown as greyed)
• Envelope doesn't stutter anymore while rotating layer (with rotation tool)
• Fixes quite a few bug fixes in 0.83 related to intiializing filter parameters (errors showing bug when asking for layer properties