2. Add `gir1.2-gwibber-gtk-0.2' as "Recommends:" of Viewer package,
and display the "Share!" button only if GwibberGtk module is present
on user's system.
This solves the above problem, as you can choose on your own
whether you are into social networking and want to use gwibber, and it
leaves the Viewer fully working if one chooses to remove gwibber from
his system. Using "Recommends:" would display `gir1.2-gwibber-gtk-0.2'
as an optional addon to the Viewer when installing it from Ubuntu
Software Center - however, it would be displayed as "Typelib file for
libgwibber-gtk2", and I doubt if anyone would guess this has to be
installed to enable social integration...