Don't hide "use customization" when editing settings

Previously, if you clicked "edit settings" in the "use customisation"
msgbox then the flow got a bit confusing, when you saved settings the
"use customisation" msgbox would have disappeared and you had to click
"next" and go back through again.  Instead, keep the "use customisation"
msgbox open when editing settings so when you save settings you are back
where you left it.

Also:
- Make "use customisation" modal so you can't click other things while
  it's open
- Separate windows in QtQuick can't be modal so we can't stop the user
  interacting with the main window while "edit settings" is open.  This
  could get confusing, so if they close the "use customisation" modal in
  the main window then close the settings window.
- Fix a bug where saved settings weren't recognised on startup, only
  after being saved during the session

src/main.qml

@@ -1208,6 +1208,9 @@ ApplicationWindow {
         onEditSettings: {
             optionspopup.openPopup()
         }
+        onCloseSettings: {
+            optionspopup.close()
+        }
     }
 
     /* Utility functions */