Add flags prop to Window (Qt only)

[maciek134]

Depends on https://github.com/kusti8/node-qt-napi/pull/7.

This is more of a conversation starter than a mergable PR, because it would probably be a good idea to add this to Wx as well (most of the useful flags should overlap, so there would only be a small translation between Qt/Wx needed), or even go higher level and add Window props like frame, dialog, etc.

Let me know what you think.