Allow editing "new position" and "search" fields

[HT-7]

Currently, pressing the left arrow key to move back the cursor erases the characters like the backspace. It would be better if the characters remained so the position can be edited.

Also, the "new position" field (Ctrl+G) should be preloaded with the current position. In hexadecimal or decimal depending on which mode is used (after #74 is implemented).

Thank you for your consideration.