Revert "Remove listeners when dropping tiles"

This reverts commit fd6aaff2f5d7d1c9e448e345d6291d311822de53.
Register change listeners for loading tiles only

With this commit we register a change listener at a time when we are guaranteed that the listener will be called. In this was we don't need to remove listeners when the tile is dropped from the tile queue. And to avoid busy-polling between IDLE and LOADING we rely on the map to call requestRenderFrame when new tiles change to LOADING from IDLE.
Merge pull request #303 from elemoine/tile

Tile loading stops after a while when there are multiple maps