Igor Kushnir authored Oct 05, 2021 and Nicolás Alvarez committed Nov 10, 2021

This change halves the number of QHash insertions, lookups and elements,
which improves performance and saves memory.

Several additional code improvements:
* remove an unused forward declaration of KTextEditor::Document;
* make local IndexedString variables const;
* Q_ASSERT that no visualizer value is overwritten during insertion.