ComboBox3.contentItem must be a QQuickTextInput to fix autocomplete

See QQuickComboBoxPrivate::updateEditText()
https://github.com/qt/qtquickcontrols2/blob/dev/src/quicktemplates2/qquickcombobox.cpp#L426

By removing the MouseArea previously in the contentItem, we lose the
ability to press+hold+drag to select a drop down item.

The wheel events still work, as they are captured by the MouseArea in
the background component.

The combobox previously closed the popup when releasing the click to
open the dropdown when editable=true.

BUG: 424076