Previously DOtherSide was an external, standalone library with own
submodule for SortFilterProxyModel. After merging DOtherSide directly
to status-desktop it should be treated as a build tool or part of build
configuration. Instead of owning c++ files by itselft, DOtherSide should
refer to all needed c++ sources in order to compile them and build
status-desktop properly.
Removing SortFilterProxyModel submodule from original DOtherSide and
changing config after merge to status-desktop was needed to be done
along with merge itself because otherwise there would be a conflict -
both DOtherSide and status-desktop had the same submodule
(SortFilterProxyModel).
Closes: #9410
Igor Sirotin
Michał Cieślak
Patryk Osmaczko
Ștefan Talpalaru