Added capability to associated views with context menues
- Context menues will be shown when the user right clicks the context
menu owner.
Small fix in lefrgithlayout
Added TopBottom layout, analogous to LeftRight layout
Added ActionOnDownListener
Convenient method for AbstractWidget
General context menues are set opaque by views
Convenient method to add loaded assets to assets
Context menu can be opaque
Merge branch 'master' of github.com:e-ucm/ead