Classlib: GUI: Reinstate proper functioning of ObjectGui system

Patched by James Harkins <jamshark70@dewdrop-world.net>

Object:gui *should* look up the implementing class by the method
'guiClass'. Commit 019586cb0ba20d7c9d851e2449dac7911bbb0952 improperly
removed this and hardcoded ObjectGui instead -- thus breaking every
class that implements 'guiClass' but not 'gui'.

The crucial GUIs are not Qt views. They are more like Controllers
in MVC. 'guiClass' here was never a redirect and it shouldn't have
been touched.
Merge pull request #1073 from marierm/fix-ObjectGui-system

Classlib: GUI: Reinstate proper functioning of ObjectGui system