Fix long freeze when reloading files

The semantic highlighting and all the extra selections create
many cursors, this does cost time in setText().
Solution: remove all extra selections on reload.

Reviewed-by: Oswald Buddenhagen
Task-number: QTCREATORBUG-1720

src/plugins/texteditor/basetexteditor.h

@@ -304,8 +304,8 @@ protected:
 
 private slots:
     void editorContentsChange(int position, int charsRemoved, int charsAdded);
-    void memorizeCursorPosition();
-    void restoreCursorPosition();
+    void documentAboutToBeReloaded();
+    void documentReloaded();
     void highlightSearchResults(const QString &txt, Find::IFindSupport::FindFlags findFlags);
     void setFindScope(const QTextCursor &start, const QTextCursor &end, int);
     void currentEditorChanged(Core::IEditor *editor);