qml: determine resize handle width according to scroll bar width

Resize handle may overlap the scroll bar. In that case, using scroll bar may be impossible because the resize handle might have a width that is larger than the width of the scroll bar.

Since scroll bar is not customized, we have no control over the width of the scroll bar. Therefore, in this patch it is proposed to set the width of resize handle to half of the scroll bar's width. This makes both scrolling and resizing feasible.