Improve consistency of settings window sidebar categories' text and icons

Summary:
This is in support of the initiative to make settings' window sidebar categories
consistent throughout KDE software by doing the following:
- Remove the word "Settings" from the category names (this is implicit, and not done in other KDE software)
- Use the app's own icon for the General category
- Use all colorful icons

Test Plan:
Before: {F7449535}
After: {F7449536}

Reviewers: #vdg, #ark, elvisangelaccio

Reviewed By: #ark, elvisangelaccio

Subscribers: kde-utils-devel

Tags: #ark

Maniphest Tasks: T10165

Differential Revision: https://phabricator.kde.org/D24270