PartsView: use a left sidebar on phone's landscape mode

We currently provide PartsView in a left sidebar only if the window's width is larger than units.gu(120). Otherwise, we position PartsView at the bottom of the window.

While this behaviour looks good on desktop or tablets, it's not suitable on phones when they're in landscape mode, since we have only a small portion of the screen on which we can display the slide.

In this case we should use a left sidebar: the size of the delegate should be equal to the size of the delegate in portrait mode.