SpinBox: enable focus on the internal TextInput

This way keyboard input works after tabbing into a SpinBox.

Change-Id: I68a482936bfed7e823002ac5e56fec4757db0ef5
Reviewed-by: Liang Qi <liang.qi@digia.com>

src/controls/SpinBox.qml

@@ -233,6 +233,7 @@ Control {
         TextInput {
             id: input
             anchors.verticalCenter: parent.verticalCenter
+            focus: true
             activeFocusOnPress: spinbox.activeFocusOnPress
             function setValue(v) {
                 var newval = parseFloat(v)