Added background panels
Fix in AddSceneElementFromResource
Added blank background to topToolbar
Added modalsContainer to Views

viewsContainer (container for differente perpsective) is different from
modalsContainer (container for context menues, that also contains the
main toolbar and viewsContainer).
Refactoring after changing Views constructor
Merge pull request #518 from anserran/backgrounds

Added some backgrounds to widgets; Fix for context menues