Opcode GUI style updates

[nashirj]

• Instruction page
  • Make compute button same size as text box
  • Add space between text box, compute button, and output status label
  • Be able to type enter in the text box to get output message (essentially have same callback for button press and pressing enter)
  • Update newline/button press callback to clear text box
  • Don't allow more than one char in text box (just check len(txt) == 1)
• Change width of buttons in dropdown to be uniform (so that no resizing)
• Update callbacks to not rerender button grid for opcode 2. just use dropdown index to determine correct callback behavior
• Clear button grid when dropdown changes (except between 0 and 1)