setting flow direction for waters

[seichter]

When entering a new waterway starting from its confluence, the initial direction is upwards.
Initially the editor cannot know what the intention of the mapper is, bs soon as the waterway tag is entered, the user should be warned, that the direction is upstream.
This would be a context sensitive warning similar to that when rotating the direction of a waterway or other direction sensitive ways.

[Don-vip]

User is "warned" through the rendering of flow arrows as soon he/she adds the waterway tag. There is a ticket (#10290) on this subject, but I do not see any reason to add an additional warning.