GCS: remove Qt5.2ism

QComboBox::currentData was introduced in Qt 5.2 and all the other
comboboxes in the gcs code use the old

	comboBox->itemData(comboBox->currentIndex());

form anyway. .

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Merge pull request #1194 from koenkooi/qt5.1

GCS: remove Qt5.2ism