Improvement: Adapt playlist cell heights to item type

[wutschel]

Currently the playlist's cell height is fixed and only supports square thumbnails. The cell height could be defined based on the item type, like it is done in "Global Search". This allows to show movie posters in their desired dimension.

[wutschel]

First impression based on keeping same thumbnail width but changed height (4:3 for pictures). I am not clear yet on how to show episodes which usually have a 16:9 thumbnail.