Store only unique strings in the QtDBus meta-object string table

Store only unique strings in the QtDBus meta-object string table

Do like moc: If the string has already been entered into the table,
just return its position, don't make a new copy. This can save
space, for example, if there are several properties of the same type;
the typename only occurs once in the string table but will be
referenced by several property descriptors.

Change-Id: I63e5c73d28ba117fd00a5261d0e89f3a3d83df9a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>