Fix scroll bar creation only for panels

Other controls like wxListBox already include scroll bars (using wxHSCROLL / wxVSCROLL), and should not create / set new ones for them
Fix paint issues on scrolled controls in wxQT

Warning reported by qt was related to trying to draw in an invalid painter: QPainter.begin(...) returned False, with the output:

QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1

This can be tested quickly with the scroll sample (nothing is drawn the QScrollArea->viewport() is used.
TLW also has a QScrollArea as central widget, so this should be solved there too.