Sidebar design changes

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

Reviewers: Elisa, mgallien

Reviewed By: Elisa, mgallien

Subscribers: mgallien, ngraham

Tags: Elisa

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