x86[-64]: Fix arch_cpu_user_strlcpy() for small buffers

The case for small buffers was broken, since scasb operates on edi/rdi,
not esi/rsi.
Add listener interface to MemoryView.

- When the target address of the memory view changes, an attached
  listener is now notified. This lets the inspector window's text input
  keep in sync with the current address when keyboard navigating the
  memory view.