Runtime type checks should only check for the mock up type, if
neccessary. If objects of another class than the mocked object itself
are processed, runtime type check should only check for the base-class
type. It should be possible to use mockup objects and non-mockup objects
within the same application.
The included headers are adapted accordingly.
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.
