Feature. UI: make word editable with on-the-fly word verification. 
- Add blinking underline cursor in the current guess to show location where new letter will be inserted.
- Update on-screen keyboard to add row with the keys: Enter, Left, Right, Backspace. Remove Enter and Backspace from existing rows.
- If current try line does not contain 5 letters, or current try is not a valid word per API call, grey out and disable 'Enter' key.
- Left and Right keys move cursor by one position.
- When cursor points to a letter, entering new letter replaces the letter under cursor, and does not change anything else.