Add special cases overriders.

Add .defs data to override GIR so we can still use our hand craft
code to handle group for Radio widgets.
Override some constructors to handle memory return value properly.
Override 'ref' method of PangoLayoutLine to avoid infinite loop.
Fix unit test: first TextTag cannot be null.
Fix constructor return value and enumerations.

Define return values of constructors as owned by the caller.
Fix enumerations parsing by getting values from the GIR data.