Cleanup in MWGui::WindowManager constructor initialzation list.

Corrected order of initialization list, and added missing
member field initializations - without this openmw crashes when
installer is interrupted and there are no valid data path
in openmw.cfg, as reported by BrotherBrick in:

https://forum.openmw.org/viewtopic.php?f=20&p=19501#p19499

Signed-off-by: Lukasz Gromanowski <lgromanowski@gmail.com>