About the trigger button. What you specify there is the name of the CSS class. aw-icon-search refers to the class in the default Aware IM CSS file AwareIM/Tomcat/webapps/AwareIM/aware-ext/resources/css/aware.css
(there are also other CSS files - depending on the theme it can be blue.css or aware-touch.css)
The CSS class must specify some icon as a background image.
You can see a list of all classes there.
You can define your own CSS file and include references to your icons (the file must be added to the startup.html)
Unfortunately, you cannot specify font glyphs as icons here...