Moving tile URL generation into separate tile source objects

CATMAID supports multiple tile types and each one has a different way of creating URLs for tiles and overviews. These changes move this functionality into so called tile sources. A tile layer will ask such an object for the file URL and the overview layer instead of creating the URL on its own. IMHO, this makes the tile layer more readable and is easier to add new tile sources.

However, up to now a tile's "alt" attribute was given the tile's base name as value. I am not sure whether this is actually needed. To me it didn't seem like it and I assign an empty string to the "alt" attribute. Any objections?

Also, I am only able to test this with the default source type (1). If you have stacks with other image types available, please test this branch. (In case you are interested in this change.)