Gives access to native window handle

A uintptr is used for the handle type, in counterpart to WId which is a
quintptr (an unsigned integer of the same size as a pointer).
Merge branch 'expose-native-window-handle' of https://github.com/oblitum/qml into win-id
Renamed WinId as PlatformId and tweaked doc.
Add smoke test for PlatformId.