when Session emits RouteAdded, each handler (editor, mixer, trigger pages etc) will
execute their callbacks in order. But Editor::add_routes() selects the routes too,
which triggers a PresentationInfo::Change signal. This is received by e.g. the Mixer
before it has even found out about the newly added stripables. This in turn
leads to severe confusion regarding the state of the selection in the mixer,
and potentially elsewhere.
So, just add a PresentationInfo::ChangeSuspender for the scope of the RouteAdde
signal emission
eba00287cd