Add some new features to the icon chooser dialog

[collinss]

• Adds a default_icon property which, when set, will cause the dialog to show a revert button. When that button is pressed, it will set the currently selected item to the default icon.
• Adds a *_add_custom_category () function which allows to specify a new category in the dialog's category list, which will (when selected) show the icons from the provided list of icon names.