Necessary for "compound" properties, where we use multiple PropertyItems for what's under the hood a single property. In this situation, when setting the value for one part of the property, the other parts of the property need to be preserved _for each element individually_, because the other parts might differ per element. Therefore, we introduce the possibility to inject a custom function to convert from QVariant to PropertyValue per element.

In 6ded409, we had made offset properties a single PropertyItem to fix some problems with applying to style. This turns out to cause other problems, because we want to be able to edit x and y values separately, also when multiple items are selected, see musescore#15452.