Grouping UI elements

Hi Caolán,

     I'm working on part of this bug: https://bugs.freedesktop.org/show_bug.cgi?id=71764. Main 
problem is that the 'Help' and the 'Extensions' buttons on the Start Center are in different 
GtkBoxes and so arrow key traversal doesn't work between them. Nested GtkBoxes are necessary for 
special effect (thin border around the buttons) which I can't replace with a working solution with 
the same appearance.
    So I tried to find a way to solve this problem:
        - First I found a "group" property with which arrow key traversal can be specified 
explicitly but it just work for RadioButtons. Maybe it can be extend to other buttons, but 
RadioButton groups has a special meaning (only one item of the group is enabled), so I'm not sure 
it make any sense for other type of buttons.
        - Second I pick up "member_of" property and Window::SetAccessibleRelationMemberOf() method 
which is used not for only RadioButtons but for other elements too. For example: 
http://opengrok.libreoffice.org/xref/core/cui/source/dialogs/hlinettp.cxx#93 (an image button 
related with a fixed text). So this property can use for other UI elements, but this relation 
doesn't affect the arrow key traversal. So secondly that came in my mind to modify the 
corresponding Window::ImplDlgCtrl() method to take account of member_of relation in case of arrow 
keys.
        - The third and simplest one to override BackingWindow::Notify() method (BackingWindow 
implements of Start Center) and handle arrow keys. I can keep at this third solution, but maybe the 
need of adding groups is not a unique problem.

    So I'd like to ask some suggestions about what would be the best way to fix this bug, in your 
opinion.


Thanks,
Tamás