Add keyboard shortcuts, disable typing

Disable typing in text field. The following keyboard shortcuts are added
to buttons:

- Number keys, dot, plus, minus, multiply, divide map to the respective
  buttons

- Return and Enter keys map to equal button

- C and Alt+C map to CE and C buttons, respectively

- ~ maps to +/- button

- I maps to inverse button

- S maps to sqrt button

- Ctrl+R, Ctrl+C, Ctrl++, Ctrl+- map to MR, MC, M+, M- buttons,
  respectively