Investigate the use of icon property of AbstractButton

As a developer, I need to know if the icon concept from AbstractButton can be used and fit with the design and concept of Neptune 3 UI. A further discussion with designer might be needed.

Previously, we didn't use the icon property since it is only released with Qt 5.10, and due to the need of using Qt 5.9, we need to drop that property and have a custom property instead. Now, with Qt 5.10, we should be able to use that property back and refactor the way icons are currently used.