Tweak sliding panel design

Some tweaks to the sliding panel design, based on some ideas from mockups: https://phabricator.kde.org/T12717#224147

• Changed outline colour of delegates to darker blue for toggled ones
• Changed the background colour of untoggled delegates to the background colour, with more opacity
• Slightly reduce padding of delegates so it doesn't look too big
• Changed the panel to be a rounded rectangle
• Add top panel labels on top of background shading when drawer is open (added after these screenshots were taken)

Current:

Tweaked (the notifications panel showing is a bug I think):