Drawer: Fix RTL by copying sizing code from upstream Default style

There was this unused property horizontal, which was clearly copied from there some time ago, and for whatever reason rewritten in anchors. Disadvantage of using anchors is that they inherit LayoutMirroring by default, unless explicitly requested not to. But Drawer::edge property is not mirrored at all. Thus, replacing custom layout code with the upstream's also fixes visual issues for right-to-left environments.

The fix works in both KF5 & KF6.

Can be tested with something as simple as this snippet: $2509