Wrap ChamplainNetworkBboxTileSource

	* champlain/champlainmm.h: Add network-bbox-tile-source.h.
	* champlain/champlainmm/.gitignore: Ignore
	network-bbox-tile-source.*.
	* champlain/src/champlain_signals.defs: Add "api-uri",
	"proxy-uri", and "state" properties.
	* champlain/src/filelist.am: Add network-bbox-tile-source.hg.
	* champlain/src/network-bbox-tile-source.[hg|ccg]: New files.
	* tools/extradefs/generate_extra_defs_libchamplain.cc: Add
	CHAMPLAIN_TYPE_NETWORK_BBOX_TILE_SOURCE.
	* tools/m4/convert_libchamplain.m4: Add conversion from 'const
	BoundingBox&' to 'ChamplainBoundingBox*'.