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.
Seth Morabito