Numbers stay in value fields

[Zverikk]

1. Create a node or way.
2. Put width=100 or any other tag on it that has number as a value.
3. Create another node/way.
4. Open "add tag" dialog.
5. There is a previously entered number as a value regardless of what tag you enter.

This behaviour is very ffrustrating. Why do I need previous tag values, if objects that I plant have more than one tag?

build 4135. This bug is probably related to #6430.