When importing from other software, it happens that weights are imported
without their type. When the user changes the type, the imported weight
is overwritten, which is not exactly a friendly behavior.
On the other hand, when changing the type after creation of a weight
entry, it is preferrable to set a default weight. This is convenient
for people who commonly use the same weight.
As a compromise, set the default weight only if it was unset. We
recognize this by a weight value of 0 g.
Fixes#2938
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
72312bec2f