qmltyperegistrar leaves out "void" return types when generating methods
and signals. Add those back when reading the files so that we don't have
to special-case "void" in the compilers.
Also, explicitly set the return type of QML signals to "void", for the
same reason.
Change-Id: I94cd9d005c458fe37160eccf5c14f02c53fc300a
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Olivier De Cannière <olivier.decanniere@qt.io>
e0dd6d0810