Don't generate spurious selection events

By calling destroy() on the active wl_data_source before replacing
it with a new one and calling set_selection() we trigger a spurious
selection(null) event before the one with the offer for the source
we are going to set.

Change-Id: I6c2f2fd029fa523312b9892c6a5050805dfa83b0
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>