Fix race condition in ListTextField

When editing a ListTextField, we would write the new value to the data point,
then emit 'root.accepted(text)'.
This causes problems for ListTextFields where the secondary text is different
to the raw value of the DataPoint, such as the generator service interval,
where the DataPoint stores the value in seconds, and the secondary text is
presented in hours, as follows (elided for brevity):

ListTextField {
    id: serviceInterval

    text: qsTrId("page_settings_generator_service_interval")
    secondaryText: Math.round(dataValue / 60 / 60)
    dataSource: settingsBindPrefix + "/ServiceInterval"
    onAccepted: function(hours) {
        setDataValue(hours * 60 * 60)
    }
}

The user might change the service interval to 1000 hours. The data point gets
set to '1000'. The 'serviceInterval' object detects that the 'dataValue' has
changed, and updates the secondary text to 1000 / 60 / 60, i.e. 0.2, which gets
displayed as 0. ListTextField continues its 'onAccepted' routine, and emits
'root.accepted(text)', which is now equal to 0. The 'serviceInterval' object
runs 'onAccepted', and calls 'setDataValue(hours * 60 * 60)', which in this
case, is equal to 0.

components/listitems/ListTextField.qml

@@ -50,12 +50,13 @@ ListItem {
 		text: dataValid ? dataValue : ""
 
 		onAccepted: {
+			let newValue = text
 			_accepted = true
 			if (dataPoint.source) {
-				dataPoint.setValue(text)
+				dataPoint.setValue(newValue)
 			}
 			textField.focus = false
-			root.accepted(text)
+			root.accepted(newValue)
 		}
 
 		onEditingFinished: root.editingFinished()

pages/settings/PageGeneratorRuntimeService.qml

@@ -56,6 +56,7 @@ Page {
 				textField.inputMethodHints: Qt.ImhDigitsOnly
 				dataSource: settingsBindPrefix + "/AccumulatedTotalOffset"
 				enabled: userHasWriteAccess && state.value === 0
+				visible: dataValid
 				textField.maximumLength: 6
 				onAccepted: function(hours) {
 					setDataValue(accumulatedTotalItem.value - hours * 60 * 60)