If a GObject is replaced by a mock up, its _get_type() function has to
be replaced as well. The replaced _get_type() functions have to return
the type of the mock up now.
A type conversion function is introduced for each GObject mockup. This
function does a runtime type check and generates a GoogleTest failure in
case of any type error. This avoids checking GObject type twice.
According to a remark in PR #738 the lower case type name is changed.
Underscores should only be used between namespace and type name and
between type name and suffixes.
