Debugger: Fix calculation in MemoryView.

- The offset calculation for mapping the current point wasn't taking
  into account the number of bytes per hex block, causing it to be
  proportionally further off if one switched to 16/32/64-bit hex mode.

Gets mouse selection working properly in said modes.
Debugger: Fix RegistersView context menu.

- Always grab the address value from the actual register value column,
  rather than the one that's in fact under the mouse. Fixes the "Inspect"
  item sending you to inspect address 0 if you happened to right click over
  the register name rather than the value.