Summary:
Design changes for the sidebar according to the mockups in the ML

## In this patch

- Make sidebar smaller
- Make the selected view use the highlight color for the text and icon
- Add a small margin to align with the views titles
- Remove backgrounds

{F5835919}

Reviewers: #elisa, mgallien

Reviewed By: #elisa, mgallien

Subscribers: mgallien, ngraham

Tags: #elisa

Differential Revision: https://phabricator.kde.org/D12715