Add more user side debugging features

[RM87]

Currently there is no way to say what went wrong when you end up with a bunch of gray tiles and some rendered ones. There should be some kind of possibility to extend the current debugging info (layers styled, map rendered, icons/text rendered).

Things that would be helpful:
*Tile number
*Whether data needed for rendering has been downloaded successfully or not

Maybe this info can be added as a half transparent layer (so that info for each tile is shown inside that tile) that can be switched on when necessary.