Additional properties of the controls can be set by using the "Advanced" section of the properties. This is useful when a particular property is not yet available in the tool for direct maintenance. Like in the example below, "iconFirst" and "activeIcon" properties are not yet available for the button control and can be added in the advanced section.
Follow the XML format in setting the values. Please refer to the SAP UI5 Controls (Samples and API Reference): https://sapui5.hana.ondemand.com/#/controls to see the additional properties of the controls.
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