Fix bug in filter function when havin multiple filters

Since commit 707323067d there has been a
bug in the SqliteTableModel::setQuery method which rendered the filters
useless when using them on multiple columns: Because this method is
called whenever a filter value is changed and because it deleted the
internal representation of the filters all filters except the one last
modified were deleted but still shown in the UI.

src/sqlitetablemodel.cpp

@@ -83,9 +83,11 @@ QString removeComments(QString s)
 void SqliteTableModel::setQuery(const QString& sQuery, bool dontClearHeaders)
 {
     // clear
-    m_mWhere.clear();
     if(!dontClearHeaders)
+    {
+        m_mWhere.clear();
         m_headers.clear();
+    }
 
     if(!m_db->isOpen())
         return;