* Enums can now be dynamically created in the _itemf callback,
using RNA_enum_item(s)_add, RNA_enum_item_end. All places asking
for enum items now need to potentially free the items.
* This callback now also gets context, this was added specifically
for operators. This doesn't fit design well at all, needed to do
some ugly hacks, but can't find a good solution at the moment.
* All enums must have a default list of items too, even with an
_itemf callback, for docs and fallback in case there is no context.
* Used by MESH_OT_merge, MESH_OT_select_similar, TFM_OT_select_orientation.
* Also changes some operator properties that were enums to booleas
(unselected, deselect), to make them consistent with other ops.
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