Element picker: do not prevent the page from receiving mouse-move-related events

### Describe the issue

Test case was [provided by a user on reddit](https://www.reddit.com/r/uBlockOrigin/comments/52ow42/how_could_i_have_found_and_blocked_this_element/):
- Visit http://reddpics.com/r/gif.
- Notice that when you hover a thumbnail on the page, a play button appears in its center.
- Launch uBO's element picker.
- Try to select the play button in any of the thumbnail: you can't.

The play button won't appear unless you hover over the thumbnail, but uBO's page wide overlay prevents the mouse move events from reaching the page, so the thumbnails end up never being hovered.