Fix WindowsRegistryKey for Qt 6.8.1 (#154)

* Fix WindowsRegistryKey for Qt 6.8.1

* Use new Qt Private API instead of restoring the old version

* Applied review changes

- Moved template specializations to header, had to add inline for this to work
- Added const qualifiers
- Added space after if
- Removed unneeded new lines
- Removed new lines inserted by the IDE
- Removed general template implementation, static asserts did not compile (due to MOC i think). Using the function with any other type than DWORD will still cause a compilation error

* Fix const

* Fix line ending