You should see the TWL site to see how the binding class/xml works. Everything is in guiTheme.xml. This is the only file that I changed.
For the button I only need a press method and addCallback method is enough to do that. Image are set because the Eforen theme is link to guiTheme.xml.
By default twl components have a theme set (a string, by example "button" for the buttons). So you can see the mapping with image and border in guiTheme.xml:
<theme name="button" ref="-defaults" allowWildcard="true">
<param name="textAlignment"><enum type="alignment">center</enum></param>
If you want to learn TWL, go the the TWL site. Mathias do a great work with this library