Added two new window attributes to set the width and height.

Externally the attributes can only be set with the global config options/
environmental variables EQ_WINDOW_IATTR_HINT_WIDTH and
EQ_WINDOW_IATTR_HINT_HEIGHT. Each overrides the default window width/height
when no configuration file is provided.

Let me know if this options should be documented somewhere, because I didn't write any documentation about there behaviour.