Misc playlist UX improvements

This should cover most of #438 except two things:

• empty view which is already covered by !798
• the removal of never played. I'm not sure we want to remove it. @mschraal what's your opinion on it?

There are other things from the mockups which are not covered yet:

• add icons in the sidebar (!801 (merged))
• add a separator before the first user playlist. Update: added
• Add a menubutton instead to SongWidget

Update 29th June:

• add a separator before the first user playlist