* refactor: add libtransmission-app
* refactor: add libtransmission-app/display-modes.h
* refactor: use app::SortMode, app:ShowMode in Qt client
* feat: add to_variant(), to_value() in serializer
* refactor: use app::SortMode in GTK client
* refactor: use app::ShowMode in GTK client
* refactor: make naming consistent with libtransmission-app