Add Tag - last entry disabled for partial dataset

[stoecker]

Introduced in #14030.

The last data entry gets disabled, e.g. when building=yes already exists. That should only be the case, when ALL elements have building=yes, not when one has it.

Example: When I select multiple ways I can't make it a building easily, when one of the ways already has the building tag.

[stoecker]

Thinking about it a bit more - On example for building=yes with multiple objects:

• When all have building: Disable
• When some have building and the building values differ: Disable --> Security mode
• When some have building=yes: Enable
• When none have building: Enable

[Don-vip]

In 11804/josm:

fix #14527 - Add Tag - last entry disabled for partial dataset