Gosu: move Window into backend-gosu/

This is a pretty big commit.  It establishes a generic GameWindow
interface that multiple backends can fit into.  The existing Gosu-based
implementation is moved into backend-gosu/, and explicit Gosu
dependencies throughout the code are much reduced.
area-tmx: Maps are editable in Tiled without editing

Until now, one had to mess with the file paths for tileset and tileset
image sources to get them to open in Tiled.  After finishes ones
changes in Tiled, one undid the path chanes for Tsunagari.  This was a
tedious process.

Tsunagari now speaks Tiled paths for tileset and tileset image sources.
Makefile: recompute depends