We need to know the target atom value to know when we need to
do something with side-effects (since side-effects are expressed via
special target values). Previously, the code side-stepped that by looking
at the data type (which was rather unique for the one side-effect
target that we supported, signalled by the TARGETS target),
but for the DELETE target that seems to be no longer an option, hence the new
field to carry this information past the convert_selection() routine.

This prevents GDK from throwing a warning when trying to convert
a DELETE target, which has no format or data objects set.

The side-effects for the DELETE target happen earlier, in GTK layer.
By the point it gets to change_property(), it's a no-op.

https://bugzilla.gnome.org/show_bug.cgi?id=786509