Draw assistant - comfortable creating of a parallel way

[Pontiac_CZ]

Often we have to draw polygon that shares part of its boundary with another previously created one. In this ticket I am discussing this shared part - at the moment we have to click every existing node on this shared way (and hope we don't miss anything which can happen very easily).
How about to introduce some sort of "draw assistant" which would activate by pressing modificator keys , let's say CTRL + ALT in add mode, and offer us drawn way along an already existing one from the last node to a spot on the old way cursor is currently touching. By clicking the user would accept this proposal and the waypart would come into effect.

[bastiK]

Closed as duplicate of #4190.