Add --changelist and --changelist-format

Allow output of changed tiles to a file specified with --changelist. The
file contains the name of each tile on a line. The path can be relative
or absolute, chosen using --changelist-format.

The code assumes that all innertiles will have been updated when a
worldtile changes, and prints the corresponding filenames to the list
directly, rather than add message passing to the innertile methods.

overviewer_core/world.py

@@ -86,7 +86,7 @@ class World(object):
         #  stores Points Of Interest to be mapped with markers
         #  a list of dictionaries, see below for an example
         self.POI = []
-
+        
         # if it exists, open overviewer.dat, and read in the data structure
         # info self.persistentData.  This dictionary can hold any information
         # that may be needed between runs.