Fixed scrollwheel behaviour in MapView

- Used the trick described on http://bdadam.com/blog/simple-usability-trick-for-google-maps.html
  Instead of ignoring scroll events when Ctrl is pressed, which would only help if we're concerned with zooming the entire page,
  we're now ignoring scroll events as long as the map was not clicked.
  Once the map is clicked or dragged, scroll events are interpreted.
  Once the user interacts with a different part of the page, scroll events are ignored again.
- fixes #26