Add new shared_paths function

[nyalldawson]

Returns a collection containing paths shared by the two
input geometries. Those going in the same direction are
in the first element of the collection, those going in
the opposite direction are in the second element. The
paths themselves are given in the direction of the first
geometry.

(Exposes the GEOS shared paths functionality for use
in expressions.)

[roya0045]

Just for info,
I did a test with the following geometries

<QgsGeometry: LineString (-0.66612641815234996 0.34521880064829824, -0.63360007212708469 0.50193693569908726, -0.3484602327425797 0.50404388032508118, -0.14424635332252822 0.81523500810372773, 0.15403925685702602 0.50775693074084183, -0.09251617095436582 0.50593509260430447, -0.15072933549432732 0.6531604538087521)>
AND
<QgsGeometry: LineString (-0.786061588330632 0.50081037277147489, 0.52998379254457051 0.51053484602917343)>

Even if the lines are snapped, an empty collection is returned. This seems to confirm that the GEOS implementation checks vertex and will not care if the lines are snapped.