Wrap ChamplainNullBboxTileSource

	* champlain/champlainmm.h: Add null-tile-source.h.
	* champlain/champlainmm/.gitignore: Ignore null-tile-source.*.
	* champlain/src/filelist.am: Add null-tile-source.hg.
	* champlain/src/null-tile-source.[hg|ccg]: New files.
	* tools/extradefs/generate_extra_defs_libchamplain.cc: Add
	CHAMPLAIN_TYPE_NULL_TILE_SOURCE.