kstyle: round corners of view item selections

what?

why?

the general idea that we should have rounded corners is mostly agreed upon by our design team and used elsewhere in plasma.

why doesn't it go all the way to the design used elsewhere in plasma?

this does not go all the way to the usual bordered look that we have in our qtquick apps due to it being technically problematic to implement qtwidgets; see !128 (closed).

TLDR of why it can't be 100% faithful to the style used elsewhere: at the bare minimum, qt is missing APIs to tell items about their surrounding selected items, which is needed to merge vertically adjacent borders to avoid a border-on-border style that is generally seen as undesirable. third-party apps also make assumptions about the format of the colours that the selection style uses, which results in them looking visually broken when we violate those assumptions. that can be pursued later, but not in this merge request.

how do i test it?

this affects qtwidgets apps using item views, such as dolphin, okular, oxygen-demo6, etc.