Wrap ChamplainNetworkTileSource

	* champlain/champlainmm.h: Add network-tile-source.h.
	* champlain/champlainmm/.gitignore: Ignore
	network-tile-source.*.
	* champlain/src/champlain_signals.defs: Add "uri-format",
	"offline", and "proxy-uri" properties.
	* champlain/src/filelist.am: Add network-tile-source.hg.
	* champlain/src/network-tile-source.[ccg|hg]: New files.
	* tools/extradefs/generate_extra_defs_libchamplain.cc: Add
	CHAMPLAIN_TYPE_NETWORK_TILE_SOURCE.