Step Count and Memory Window enhancements

Introduces several changes requested by Mario Keller. The simulator now has a variable step count that can be selected by a drop-down box on the main window. This change also displays ASCII characters in the Memory window.

src/main/java/com/loomcom/symon/Preferences.java

@@ -29,16 +29,12 @@ public interface Preferences {
 
     public static final int DEFAULT_PROGRAM_LOAD_ADDRESS = 0x0300;
 
-    public static final int DEFAULT_BORDER_WIDTH = 10;
-
     public static final boolean DEFAULT_HALT_ON_BREAK = true;
 
     public JDialog getDialog();
 
     public int getProgramStartAddress();
 
-    public int getBorderWidth();
-
     public boolean getHaltOnBreak();
 
     public void updateUi();