Rework OS Customization

In the new flow, it doesn't make sense to _not_ save the OS
customization parameters, so remove the ComboBox.

Additionally, our data model was failing to notify the UI of changes to
the saved settings state. Due to time constraints, I'm not able to
resolve the binding in the 'correct' manner, but I can introduce a
makeshift status signalling mechanism to prevent UI inconsistency.

src/main.qml

@@ -1183,6 +1183,10 @@ ApplicationWindow {
 
     OptionsPopup {
         id: optionspopup
+        onSaveSettingsSignal: {
+            imageWriter.setSavedCustomizationSettings(settings)
+            usesavedsettingspopup.hasSavedSettings = true
+        }
     }
 
     UseSavedSettingsPopup {
@@ -1196,6 +1200,8 @@ ApplicationWindow {
             confirmwritepopup.askForConfirmation()
         }
         onNoClearSettings: {
+            hasSavedSettings = false
+            optionspopup.clearCustomizationFields()
             imageWriter.clearSavedCustomizationSettings()
             confirmwritepopup.askForConfirmation()
         }