Fix uses of deprecated Qt methods

There are still a few leftovers from the Qt 3 to Qt 4 migration that can
be found by defining QT_NO_DEPRECATED. Here is a summary of the changes:

* QDir::convertSeparators was changed to QDir::toNativeSeparators with Qt 4

* QRexExp::numCaptures was changed to captureCount with Qt 4

* QTextIStream/QTextOStream was merged into QTextStream with Qt 4

* QFileDialog::selectFilter/filters was changed to
  selectNameFilter/nameFilters with Qt 4.4

* qVariantCanConvert/qVariantValue was changed to canConvert/value with Qt
  4

Note that if a conversion to Qt 5 will happen someday, this will have to
be done since Qt 5 removes these deprecations.