Manually config tileSize

Dealing with a manually entered `tileSize` is actually easier to reason about,
therefore we no longer calculate it based on `tileCount.`

src/tile-component.coffee

@@ -21,6 +21,7 @@ class TileComponent
     @screenRowsByLineId = {}
     @lineIdsByScreenRow = {}
     @domNode = document.createElement("div")
+    @domNode.classList.add("tile")
     @domNode.style.position = "absolute"
     @domNode.style.display = "block"