Add example that hosts Qt Quick inside a widgets UI

Must demonstrate

• focus handling
• layout integration
• shortcut handling
• clipboard (copy and paste)
• consistent native style look and feel

May demonstrate

• drag and drop
• transferring data between widgets and Qt Quick

There is already the example qtdeclarative/examples/quick/embeddedinwidgets. We can extend that example. If the example becomes too complex, maybe consider splitting it.