Represents a button control to be displayed in the AppBar.
AppBarButtons can only be added to a Page's PrimaryCommands & SecondaryCommands. At this time, they are only support on ContentPages.
AppBarButtons in the PrimaryCommands collection will appear in the actual AppBar. They will display either Text or Icon.
AppBarButtons in SecondaryCommands collection will appear in the AppBar menu with 3 dots. They will only display Text in the menu.
|Icon||The icon for the control.|
|String||A string that specifies the text contents of the button.|
|Occurs when the button control is clicked.|