Better handling of semicolon in values

[skyper]

The handling of semicolons could be enhanced. For background please have a look at:

• ​thread at talk@osm
• ​Jochen's blog an analyses

Two points came to my mind:

1. make it harder to add semicolon separated values when combining by asking the user for interaction for each value with conflict and not automatically adding both values with semicolon as separator.
2. Validator warnings/errors about semicolons in values where it makes no sense as many keys need single value to work.