[BasicListItem] Allow overriding vertical fill by setting height
requested to merge ngraham/kirigami:basiclistitem-verticality-improvification-maneuverment-operationalism into master
This MR makes Leading and Trailing items of BasicListItem have an implicitHeight equal to the parent's height, to preserve the current vertical fill feature which many apps are relying on. However if this is undesirable, you can simply specify the height manually and the item will use that height and be vertically centered in the list item instead.
Documentation and tests are updated to reflect this.
This is an alternative to !295 (merged)