Allow custom saving/loading of SettingsPage autowidgets

Allow custom saving/loading of SettingsPage autowidgets

SettingsPages can automatically handle child widgets' contents (e.g. track changes,
load and save) if those have the properties "settingsKey" and "defaultValue" set. However,
sometimes widgets need to be saved to something else than a Settings object (e.g. a core config object).

You can now leave the settingsKey property empty for your autowidgets and reimplement
{save|load}AutoWidgetValue() in your derived SettingsPage to handle custom saving and loading.