This is mostly a simple lexical search+replace but the absence of operator< for
std::weak_ptr<T> leads to some complications, particularly with Evoral::Sequence
and ExportPortChannel.
During drag-motion callbacks the data to be dragged is n/a.
However we like to discriminate if drop is possible.
When dragging regions, the data-type of the region to be dragged
is unknown, so different `x-ardour/region` targets are not an
option, either.
Until a better option is presented, a static global is used
to set the data-type for region.pbdid drags.
