LRN authored Feb 08, 2019 and Philip Withnall committed Feb 13, 2019

G_RESOURCE_OVERLAYS is a list of resource-path and filesystem-path pairs.
Since on Windows filesystem paths use ':', this list can't be ':'-separated
there. Fix that by making it ';'-separated on Windows. Make the parser
error clearer (we're not looking for a slash, we're looking for an absolute
path).