Design tweaks for the PlayerBar

Summary:
Design changes for the elisa player bar according to the mockups in the ML

Included in this patch

• Move menu button to the bar
• Make volume slider a bit smaller
• Move repeat and shuffle to the bar
• Fix the seek's handle so it doesn't show the radius of the elapsed rect
• Make player bar slightly thinner
• Make background of the seek/volume sliders darker so they are more visible
• Make volume button closer to slider
• Tweak length of seek bar and spacing between elements to make the design feel more balanced
• Align "remaining tracks" text and imported notification with the hamburger menu

Notes

• I haven't tested this with a RTL layout. I follow the LayoutMirroring pattern so it should be mostly ok (plz test!)
• I'm pretty sure I broke something when I moved the shuffle and repeat buttons to the bar because their state isn't being saved when I restart Elisa, but I can't figure out what it is.
• Do we have another choice of next/prev track icons? The vertical bars on these are super huge and make them feel way too heavy. Specially compared to the shuffle/repeat icons

Reviewers: mgallien, Elisa

Reviewed By: mgallien, Elisa

Subscribers: ngraham, mgallien, astippich

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D12714