LSP formatting is canceled during save

Resulted from https://codereview.qt-project.org/c/qt-creator/qt-creator/+/647402

The language client formatter is incorrectly canceling formatting
requests when invoked from "Auto Format on Save" because of the logic
in the handler of QTextDocument::contentsChanged signal. This signal
is emitted both when formatting begins/ends and when the document is saved
and this created a condition where formatting would work when manually
triggered via context menu but fail during auto formatting on save.

We need a mechanism that differantiates user edited changes and document changes after saving.