pxtroute: current 'export' button approach upsets popup blockers

[jaustin]

The current approach for the export button on the 'pxtroute' branch tries to pop up a window.

This appears to snag the popup blocker on Firefox and Chrome, and so means the first expereince the user has of a dialog that says "a window should have opened" but probably it hasn't.

I think this could easily be resolved like this:
Modify the dialogue to say something like
"Export will take your script and import it into the editor at Microsoft MakeCode. It will be automatically upgraded to be compatible. <something about data sharing and how MakeCode behaves"

Then have an 'Open' and 'Cancel' button exactly as they are today.
@pelikhan did you encounter the popup button?