Allow custom ordering of waypoints

[koebi]

When requesting routes using the Directions -> Points (1 Layer) or Directions -> Points (2 Layers) processing tools, the Feature ID of the layer's features are used to determine in which order waypoints are traversed.

Since the feature IDs are not manually changeable, this means that there is no way to change this order from within QGIS¹.
This could be fixed by adding an option to specify a field from the layer that is used for ordering.

¹: except for creating a new layer with the desired ordering