Use svg for tag icons
Test Plan:
rm -rf ~/.cache/icon-cache.kcache and delete old icons in ~/.local/kde/share/icons/hicolor/16x16/actions/tag_*.png (not the checkbox)
Reviewers: niccolove
Reviewed By: niccolove
Subscribers: IlyaBizyaev
Differential Revision: https://phabricator.kde.org/D29205