QAbstractItemView, style sheet vs model: padding-left requires border implies color overrides BackgroundRole

A style sheet block `QAbstractItemView::item` causes the following cascade of issues:

• `padding-left` is ignored unless a border as is set,
• as soon as there is a border property, the style sheet takes control over colors,
• colors set for the BackgroundRole in the model's data() function are henceforth ignored.

The overriding of model settings by style sheet settings is such a long-standing problem that it must be considered no longer a bug, but a feature. A largely undocumented feature though. So I'd just suggest some explanation in the docs.

Furthermore, I'd ask whether it is possible to break the above cascade at some other point:

• make `padding-left` work even if there is no `border` property,
• if the style sheet specifies border width and style, but not border color, then do not take over control control.