playertoolbar ux improvements

This is based on the new mockups.

Initial MR

Some remarks and questions (Hence the WIP state)

1. I'm not sure about the new art size. This has increased from 34x34 to 68x68
2. The repeat button is missing from the new mockups but we still need it (at least at the moment). How should we handle it?
3. I've added the flat style to the buttons. Therefore, the borders are now visible on hover. It's not obvious from the mockups if that's the desired effect
4. I'm not sure how to handle the margin between the song info and the progress scale

Here is the current result

cc @aday

Update 28 April

main changes:

• improved vertical spacing between the controls (ArtSize is now correct)
• play/pause button style now respects the mockup (relief)
• no border on hover for the prev and next button
• added some horizontal spacing (4px) between the prev/play/next buttons

Update 3 May

• control buttons and progress scale are now centered