EffectControlDialog: Use 'Preferred' size policy

This does not have any effect, as the surrounding MDI sub window always
uses the minimumSizHint. It's just preparation.

src/gui/EffectControlDialog.cpp

@@ -36,6 +36,7 @@ EffectControlDialog::EffectControlDialog( EffectControls * _controls ) :
 	m_effectControls( _controls )
 {
 	setWindowTitle( m_effectControls->effect()->displayName() );
+	setSizePolicy( QSizePolicy::Preferred, QSizePolicy::Preferred );
 }