Reuse existing enums instead of expanding them since it bloats the binary.
The icons enum for example contains over 900 items and was being
expanded 17 times (once for each function that takes an icon argument).
Similar with the event type enum which contains over 200 items and was
duplicated 7 times.
makesrna.c now matches enum definitions from declarations in
RNA_enum_items.h, using their identifiers when found.
The overall space saving on my system is 776kb
(tested with a stripped release build).
Reviewed By: brecht
Ref D12245
