Inspector offset property: fix editing x and y value individually when multiple elements are selected with potentially different values for x and/or y

[cbjeukendrup]

Resolves: #15452
Res0lves: #15752 (already fixed meanwhile)

To fix issue #15452, we need to do two things:

1. Create separate property items for x and y (and do something ugly to make resetting and applying to style work correctly)
2. Create a special setter function that work per element, so that if you set the x value, for each element individually the y value will be preserved.

So we need to add another customisation point to buildPropertyItem.

We can also see that different properties need different kinds of customisation possibilities:

• some need nothing
• some only need to specify a simple element-agnostic function to convert from UI value to internal PropertyValue
• some need to specify an element-specific function to convert from UI value to internal PropertyValue; in those cases we need to force the caller to specify a way to convert to a style value too, which solves #15752.
• some need to specify a fully custom setter function; often they only need to call some extra method after calling the default behaviour, so we add a convenient way to retrieve the default function.

In this PR, I added all these ways. I also eliminated an unnecessary conversion from QVariant to QVariant, by allowing to specify a direct conversion between PropertyValue and QVariant.

[cbjeukendrup]

Finally rebased this dreadful PR. After looking at it again, I think it's not that dreadful at all; it may look a bit complicated, but it isn't really. Also, the complicatedness is mostly confined to AbstractInspectorModel; any attempt to simplify that, would result in some awkwardness elsewhere, so it's a bit of a trade-off.
However, there are still some refinements to be made, so I'm marking it as draft.