Vector data importer: add "edit pipes" functionality

[leendertvanwolfswinkel]

• Add an option "edit pipes" to importers for weir and orifice (not relevant for culverts)
• If a weir or orifice point is on a pipe (or snapped to the pipe), it can be handled a bit more simply than for channels; just find if it is closer to the start or to the end of the pipe, and insert the weir/orifice there. If the length of the weir/orifice is more than the length of the pipe, the entire pipe can be replaced.