Don't adjust the sidebar size with the window size, and reduce the text size a bit

Summary:
Currently the sidebar text is huge and the sidebar itself adjusts its width with
the window's own width. This results in sidebar items' text getting elided with
most window sizes.

This patch reduces the text size a bit and fixes the size, resolving both problems.

BUG: 406481

Test Plan:

Reviewers: mgallien, Elisa

Reviewed By: mgallien, Elisa

Subscribers: filipf

Differential Revision: https://phabricator.kde.org/D20577