title: Change QSortFilterProxyModel behaviour for multiple column filtering
body: <p>We have a <code>QSortFilterProxyModel</code> installed on a <code>QTableView</code> and two (or more) <code>QLineEdit</code> for filtering the view (based on the text of these <code>QLineEdit</code>s)</p>
  <p>In our view we have a slot that tells us the string of lineedits and the current column that we want. Something like this :</p>
  <pre><code>void onTextChange(int index, QString ntext) {
      filter.setFilterKeyColumn(index);
      filter.setFilterRegExp(QRegExp(ntext, Qt::CaseInsensitive));
  }
  </code></pre>
  <p>On the first column we have names in the second we have year of birthday.</p>
  <p>Now we enter a year for column 2 (for example 1985). Until now filtering is ok but when we switch to the first lineedit and enter a name (for example john) the previous filtering based on year will reset.</p>
  <p>How could we change this behaviour for our custom <code>QSortFilterProxyModel</code> ?</p>
  <p>(Actually when we change the filter keycolumn the filtermodel must filter existing view not reset it)</p>
  <h1>Update...</h1>
  <p>Based on @Mike's answer :
  If you interacting with unknown column count using <code>QMap&lt;int, QRegExp&gt;</code> will help you</p>