Preliminary support for Polylines.

So, as of this commit, there is Polyline support in the file format;
which means that Polylines can be loaded and saved.  They are also
displayed properly in the GUI, but you can't interact with them as of
yet other than selecting them with a selection rectangle and deleting
them.  The precision of the file format has been expanded from 8
decimal places to 16; also, the adding of Polylines did not necessitate
bumping the format's version.