C++ editor: Drop extra selections before large editing operations.

Updating the text cursors that the extra selections are based on can
get quite slow if there are changes in a lot of positions - like when
reindenting a file.

Dropping some text cursors can increase performance significantly in
these cases.

Done-with: mae

src/plugins/texteditor/basetexteditor.h

@@ -500,6 +500,8 @@ private:
     void updateHighlights();
     void updateCurrentLineHighlight();
 
+    void maybeClearSomeExtraSelections(const QTextCursor &cursor);
+
     void drawFoldingMarker(QPainter *painter, const QPalette &pal,
                            const QRect &rect,
                            bool expanded,