LanguageClient: Improve formatting

Use Utils::ChangeSet and TextEditor::RefactoringChanges to apply the
text edits that are reported from language server to format the current
file.

Change-Id: Id2f490b6e2791f676ebc751219bfbbf9e178f120
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/texteditor/textdocument.h

@@ -101,6 +101,7 @@ public:
 
     void setFormatter(Formatter *indenter); // transfers ownership
     void autoFormat(const QTextCursor &cursor);
+    bool applyChangeSet(const Utils::ChangeSet &changeSet);
 
     TextMarks marks() const;
     bool addMark(TextMark *mark);