Drop Qt4 support

This commit removes all code and configuration that was required for
building with Qt4. Hopefully noone really needs it anymore.

See issue #603.

src/EditTableDialog.cpp

@@ -277,11 +277,7 @@ void EditTableDialog::itemChanged(QTreeWidgetItem *item, int column)
             if(item->checkState(column) == Qt::Checked)
                 pk.push_back(field);
             else
-#if QT_VERSION_MAJOR >= 5
                 pk.removeAll(field);
-#else
-                pk.remove(pk.indexOf(field));
-#endif
 
             if(item->checkState(column) == Qt::Checked)
             {