This commit adds a virtual method to QgsMapLayer to enable the creation of custom
elevation profiles for any type of layer.
The methods returns a pointer to a helper class of type
QgsAbstractProfileSource which is a factory for profile generators
(cf. documentation for QgsAbstractProfileSource).
Existing layers that derive from QgsAbstractProfileSource just override
this method returning a *this* pointer.
As the method is *sipified* it can be used in python to declare custom
profile source for classes derived from PluginLayer (ownership of the
QgsAbstractProfileSource must then be managed on the python side).
