Update ext_qt to Qt 5.12.1 on Windows target
The follwoing patches has been dropped:
- private headers (I'm not sure it is still needed)
- invisible panels in fullscreen mode (it looks like it works fine now)
- windows store wrap crash (already in 5.12.1)
- QTBUG 57299 (QTemporaryFile problem) (already in 5.12.1)