I've rebased the branch to current master. It still works fine.

*** Bug 751869 has been marked as a duplicate of this bug. ***

After looking at your branch, I made a couple changes:

  o Removed duplicate declaration of displayable values for
    GtkSelectionMode, these were already declared by GtkListBox

    Explanation: The enum type displayable values should really
    be parsed before all the widget types, it's something we never
    got around to.

    So instead, the first widget class which uses an enumeration
    or flags type property is responsible for declaring the
    translatable displayable values for that type.

    Note that Glade issues helpful warnings on the console so
    that you can catch these errors in development, they look like:

    GladeUI-WARNING **: Already registered displayable value Start for start (type GtkPackType)


  o Removed duplicate declaration of GtkListBoxRow (note there is also
    a warning issued by Glade at startup time to help you catch this).


There is also a minor undo/redo related bug which occurs consistently when using the "Insert Before" packing action (which is only accessible when right clicking on the GtkFlowBoxChild in the Inspector view).

The GtkListBox implementation shares this flaw, and I have filed that as bug 759715.

The basic functionality is good, merged.