Refactor: simplify NoteContent

Summary:
Small refactor to reduce the size of NoteContent class

Move cssColorsName() to Tools, and also use a simpler QHash
Move loadNode() to NoteFactory

Test Plan: It works just the same

Reviewers: alex, ognarb

Reviewed By: alex, ognarb

Differential Revision: https://phabricator.kde.org/D29686