First draft of a playlist layout config editor.
It is currently working, but missing a lot of features suppored by the playlst rendering, such as individual element sizes and layouts, so it is not super usable yet.

I also cannot save layouts, but it will load the setup of the selected layout (click the "edit" icon in the layout selection combobox to activate it) and pressing the preview button will make the playlist display the currently defined layout.

Lots of work to be done on this one still, so no bug reports just yet please!