Implement Step-Over functionality

This change adds a new "Step Over" button, which is enabled
when a "JSR" (Jump to Subroutine) instruction is encountered
while stepping through execution one instruction at a time.

Pressing "Step Over" will continue running instructions until a
corresponding "RTS" (Return from Subroutine) is encountered.
While stepping over a subroutine, the call stack depth is tracked
in an attempt to halt only when the *correct* RTS is encountered.
In other words, it should skip over other subroutines nested
inside the call you want to step over:

```
     JSR A # Step over here...
     ...
     JSR B
     ...
     RTS   # Returns from 'B', doesn't halt
     ...
     RTS   # Returns from 'A', halts after return
```

Implements feature request #18.

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

@@ -1,6 +1,6 @@
 /*
- * Copyright (c) 2016 Seth J. Morabito <web@loomcom.com>
- *                    Maik Merten <maikmerten@googlemail.com>
+ * Copyright (c) 2008-2025 Seth J. Morabito <web@loomcom.com>
+ *                         Maik Merten <maikmerten@googlemail.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files (the
@@ -22,7 +22,6 @@
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 
-
 package com.loomcom.symon;
 
 import com.loomcom.symon.machines.*;