Snap new entrances to building contours

[Zverik]

When an entrance is placed <10m from a building contour, merge it, adding the node to the way. There should be two changes in the pending uploads: for the entrance and for the building.

[Zverik]

Actually we can do this without downloading all building contours. Just store the "snap to contour" flag in an OsmChange and after uploading everything, but before uploading entrances, download all building contours in a small radius for all entrances. And snap then.

The drawback would be the absence of an address hint in the snapping checkbox. We wouldn't know where the entrance will be snapped until after uploading.

[Zverik]

Done in 0.1.6.