ComboBox style: Remove old styling API in Base pop-up frame

Also, make it more useful while not yet implemented.

Change-Id: I954623c83e24398e0b4d099c1ab3f608082c2073
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
ComboBox style: Remove old API in desktop drop-down frame

The contentWidth, contentHeight properties no longer exist. We should
use padding instead.

Task-number: QTBUG-38054

Change-Id: I0e2574a5b17e12b9e43a3ab42e86ce38a31fff84
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
ComboBox: Update currentText when currentIndex goes from -1 to 0

We need to help things a bit because the internal selectedText property
won't change in that specific case.

Task-number: QTBUG-38036

Change-Id: Id997da97a77faad8ffaad08e8b9c7614d8b6b7fc
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>