Link up the evaluation and the UI buttons.

src/main/resources/abacus.fxml

@@ -4,6 +4,7 @@
 <?import javafx.scene.layout.*?>
 <?import javafx.geometry.Insets?>
 
+<?import javafx.scene.text.Text?>
 <BorderPane xmlns="http://javafx.com/javafx"
             xmlns:fx="http://javafx.com/fxml"
             fx:controller="org.nwapw.abacus.fx.AbacusController">
@@ -15,9 +16,10 @@
                         <VBox>
                             <ScrollPane prefHeight="50" vbarPolicy="NEVER">
                                 <padding><Insets top="10" bottom="10" left="10" right="10"/></padding>
+                                <Text fx:id="outputText"/>
                             </ScrollPane>
-                            <TextField text="Input"/>
-                            <Button text="Calculate" maxWidth="Infinity"/>
+                            <TextField fx:id="inputField" onAction="#performCalculation"/>
+                            <Button fx:id="inputButton" text="Calculate" maxWidth="Infinity" onAction="#performCalculation"/>
                         </VBox>
                     </bottom>
                 </BorderPane>
@@ -25,5 +27,5 @@
             <Tab text="Settings" closable="false"/>
         </TabPane>
     </center>
-    
+
 </BorderPane>