Fix disappearing + in debugger in dark mode

Simply remove the background from the sidebar displayed behind the + buttons.
This was being explicitly set to white, while the + buttons change to white
in dark mode, so they were invisible. I tried using
`Display.isSystemDarkTheme()` to choose a different background, as well as
adding a border to it, but the minimal display really looks best. We allow
the canvas background to adjust to the color of the window, as the +'s do,
always giving good contrast and no visual noise.

Partial fix for #352.

Java/Debugger/src/edu/umich/soar/debugger/helpers/FoldingText.java

@@ -26,11 +26,7 @@
 import org.eclipse.swt.graphics.Rectangle;
 import org.eclipse.swt.layout.GridData;
 import org.eclipse.swt.layout.GridLayout;
-import org.eclipse.swt.widgets.Canvas;
-import org.eclipse.swt.widgets.Composite;
-import org.eclipse.swt.widgets.Event;
-import org.eclipse.swt.widgets.Listener;
-import org.eclipse.swt.widgets.Text;
+import org.eclipse.swt.widgets.*;
 
 import edu.umich.soar.debugger.modules.AbstractView;
 
@@ -773,8 +769,6 @@ public FoldingText(Composite parent)
         m_Text.setLayoutData(data2);
 
         m_IconBar.addPaintListener(this::paintIcons);
-        m_IconBar.setBackground(m_IconBar.getDisplay().getSystemColor(
-                SWT.COLOR_WHITE));
 
         m_IconBar.addMouseListener(new MouseAdapter()
         {