SOLIDUI: Customised Settings widget on AppBar

## Description

Add a parameter `settingsWidget` to SolidScaffold, which allows developers to define their own SETTINGS widget and pass that through to SolidScaffold(settingsWidget=MySettings()). If it is defined then add it to the APPBAR next to ABOUT. If not provided then no SETTINGS button in APPBAR (or MENU rail).

## Why

This will make the Settings widget on AppBar more flexible.