Another attempt to fix focus problems with Display.setParent on Windows:

- Reverted to WS_CHILD in window flags.
- Added WM_MOUSEACTIVATE handling: Focus the display + return MA_NOACTIVATE.
- Removed unnecessary appActivate calls in WM_ACTIVATE. We don't receive WM_ACTIVATE with WS_CHILD and the focus events do the job just fine.
- Detect parent window focus in update and force focus the display. This is useful when we alt-tab to the parent window and we don't receive any events.