sdbus++: convert binding enum types to valid C++

When an enumeration is used in the bindings, the type of
it depends on the context.  We use 'string' types to send
the enumeration across the dbus, so we need to use 'string'
types when doing message operations.  We use the real enum
type when calling binding functions, so we need to specify
that in prototypes.

Change-Id: I31ab137b0c751d8a2bdec1c417a60455690ce103
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
5 files changed