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We can't break HashTable foreach, so implement our foreach with a HashTableIter
instead of calling GLib's so that the table may be modified while iterating
and iteration stopped immediately.

This gives better consistency by having tile and pawn related signals
next to each other, as well as making the board more self-sufficient
with it getting signals for all changes on it.

This field scoring implementation seem to work (in spite of hacks)
but need more testing and optimisation.

Warning: only basic testing was done as complicated cases could not
be triggered easily. Waiting for the GTK client to test.

This is not a very good design but we have several bad reasons to do
this, so…

is_tile_placeable didn't duplicate the tested tile so it left it with
the last rotation angle checked.

This is needed to allow null arguments as output variables without
crashing. It also improves the HashTable API.