* Made it based on string lookups rather than fixed enum, to make
it extensible by python scripts.
* Context callbacks now also have to specify RNA type when returning
pointers or collections. For non-RNA wrapped data, UnknownType can
be used.
* RNA wrapped context. The WM entries are fixed, for data context
only main and scene are defined properties. Other data entries have
to be dynamically looked up.
* I've added some special code in python for the dynamic context
lookups. Tried to hide it behind RNA but didn't find a clean way to
do it yet. Still unused/untested.
* Also minor fix for warning about propertional edit property in
transform code, and fix for usage of operator poll with checking if
it was NULL.
