Validate population key

[naoliv]

It seems that JOSM doesn't validate the population key. Anything can be entered as value and no warning is given.
JOSM should accept only integer numbers as value.

Something like this, maybe:

*[population !~ /^[0-9]+$/ ] {
        throwWarning: tr("{0} must be an integer value", "{0.key}");
}

[simon04]

In 6769/josm:

fix #9631 - Validate population key - patch by naoliv