Add ENUM type to REPC

This will allow enums/Q_ENUM (or Q_ENUMS, pre-Qt 5.5)
to be generated by REPC so enumeration types can be
easily passed over the QtRO network.  Enums are put
in their own Q_GADGET class.

Change-Id: Ic93ed5533c9c3c6516be4c9a1b04ce188803e014
Reviewed-by: Continuous Integration (KDAB) <build@kdab.com>
Reviewed-by: BogDan Vatra <bogdan@kdab.com>