sdbus++: complete server support for enumerations

Perform final type conversion using 'convert*' functions to change
between dbus strings and C++ enumerations for methods, properties,
and signals.

Change-Id: I22e8fd424eeecac176c64fcc3e7a5d3caf05804a
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
4 files changed