Open app settings page

### Feature description

If a user has deliberately denied access to a permission, the result of [requestAuthorization](http://docs.tabris.com/latest/api/permission.html#requestauthorizationpermissions) could be `denied` or `rejected`. If the user subsequently changes their mind and wishes to allow the permission to the app, they would need to proceed to the OS' settings page for that app to grant the permission. In this case it is [recommended](https://developer.apple.com/design/human-interface-guidelines/ios/app-architecture/settings/) that instead of showing an error message that the app should open the settings page directly for the user so they can grant the permission. Currently, it is not possible to open this page.

### Proposed solution

Provide access to the [`openSettingsURLString`](https://developer.apple.com/documentation/uikit/uiapplication/1623042-opensettingsurlstring) and [`ACTION_APPLICATION_DETAILS_SETTINGS`](https://developer.android.com/reference/android/provider/Settings#ACTION_APPLICATION_DETAILS_SETTINGS) strings if they could be used with [`app.launch()`](http://docs.tabris.com/latest/api/app.html#launchurl) or provide a new method e.g. `app.launchSystemSettings()`.