Way Splitter - breaks relations when way is incomplete

[Larry0ua]

Steps to reproduce the bug:

1. Download relation 1742393.
2. Select any node that is in the middle of the way.
3. Press P to split way.

Observed behavior:
way is splitted

Desired behavior one of the following:

• message box is shown saying "You are going to split way that is not downloaded completely. That may break relations using this way. Download way completely before splitting?" with buttons "Download" (green download icon), "Split without downloading" (no icon), "Cancel" (red cross icon - cancel default)
• some epsilon bounds around the point is downloaded and then way is splitted (not really preferred way to solve the ticket as is not obvious to the user)