qml: create the scroll bar depending on the orientation in `ListViewExt`

By default we expect the viewport to cover the content width or height depending on the orientation, so in normal cases we only need one scroll bar. This behavior can obviously be overridden where the list view is reused.

This fixes multiple things:

• Not being able to interact with the bottom part of the list view even if the orientation is vertical and the viewport covers the whole content width, because the scroll bar may still be technically visible and consume events even though it does not display anything visually.
• Flashing scroll bar upon creation of the view until the content and viewport sizes are determined (regarding the orientation where scroll bar is not wanted).

Request review @chub.