winex11: Misc window config code tweaks.
Progressing towards !6569 (merged), and instead of dropping the old fields (!6731 (closed)) now, which could be considered riskier.
- Avoid requesting unnecessary window config / state changes,
- avoid triggering spurious window restore by updating _NET_WM_STATE on iconic windows,
- simplify some code flow in preparation for delayed Win32 window state updates.