Support for breakpoints

- Adds a new window that allows adding and deleting breakpoints.
  Will halt the simulator when a breakpoint is reached.

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

@@ -911,13 +911,6 @@ public class Cpu implements InstructionTable {
         this.clockPeriodInNs = clockPeriodInNs;
     }
 
-    /**
-     * @return The simulated clock period, in nanoseconds
-     */
-    public long getClockPeriodInNs() {
-        return clockPeriodInNs;
-    }
-
     /**
      * Return the current Cpu State.
      *