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fixes https://git.goltzsche.net/caosdb/customers/glaz_awi/ext-awi/issues/32

Previously, drop listeners could be assigned multiple times. This under
some circumstances lead to the corresponding function be called multiple
times on one drop which resulted in the above mentioned bug.

Now, the named functions are directly supplied to addEventListener.
Mulitple calls no longer occur.

At the same time I made listeners uniform, i.e. all are part of the
edit_mode namespace and all are provided directly (without passing it to
e.g. set_entity_dropable)