Windows: Correctly associate IME contexts.

Associate a 0-context with the window if IME is disabled, store
this state as a flag to QWindowsWindow. Associate default context
again when enabled window gains focus.

Task-number: QTBUG-40691
Change-Id: I78d5494a05f93a39e245ca7c096d45445e684ea8
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>