Task panel visual improvements

Implement task panel mockups.

TODO:

• Move icons to breeze icons (icon paths are hardcoded right now) https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/127
• Fix icon colours when button colour changes
• Fix icon colours when button is disabled

Tasks:

• Use cuter icons
• Reduce size of icons in comparison to panel
• Use 80% of available width so buttons are slightly closer together
• Refine task button opacity animation so that it is fully played on a tap (currently there appears to be no tap feedback because the animation duration is so low)

Mockup:

Actual: