Fix backgrounds and layouts of delegates
Note: Draft because it is to be merged after !14 (merged) lands (it is based off that).
SwipeListItem: Drop custom background styling in favor of standard property
Using explicit Kirigami.Theme.* attached properties is, unfortunately, still required because of BUG 459071.
Replace transparent Rectangle with plain Item in delegates
Fix layouts of delegates, remove unnecessary clipping
Use of clipping where it is not needed worsens batching of scene rendering, making it less efficient and more memory consuming.
Also, all RowLayout and ColumnLayout layouts should override default spacing (5) with a standard Kirigami value.