LPS-51006 remove unused variable
LPS-51006 get menu items from display context
LPS-51006 add factory method for media gallery

The set of visible menus in media gallery is different than  those of
DL. Expose two factory methods in the context factory, so that DL and MG
can request the appropriate display context.
LPS-51006 extract base class

Create two display contexts (DL and IG) and make both extend a new base
class.
LPS-51006 change add*Menu methods visibility
LPS-51006 build menu items in subclasses
LPS-51006 call self method
LPS-51006 Use IG (Image Gallery) because that's the term we use in source code to refer to Documents and Media Gallery
LPS-51006 SF