Debugger: Add selection support to MemoryView.

- The Inspector's memory view now supports selecting chunks of the hex display in
  the manner one would in a TextView. The selection can also be copied to the clipboard,
  or if it matches the size of a target address, can be used as input for an address to
  inspect directly.

Still needs some fine tuning, but basically works.
Build fix.
Debugger: Fix data copy issue.
- B_PRIx8 unfortunately doesn't necessarily strictly stay within the bounds
  of an 8-bit hex constant, so force the size passed to snprintf to make it
  that way.