Add date separator in timeline

[bwindels]

It'll be a bit tricky, but we should really do the proper thing and have date separator be represented by their own tile in TilesCollection, rather than a flag on other tiles (which would be messy to implement in the view).

Tiles could signal that a tile should be inserted when setting the sibling, OR, that the tile itself should be removed. This way, a DateTile can be inserted where we go from one day to another, and the DateTile can remove itself if it stops having siblings of different dates.