Match xterm behaviour when pressing Ctrl+Backspace

Summary:
Change the default keyboard translator to match xterm behaviour:

• Pressing Backspace without any modifiers sends '^?', ASCII DEL character
• Pressing Control+Backspace sends '^H', BS character

BUG: 391075

Test Plan:

• Before applying the patch pressing Ctrl+V (to insert verbatim) then Backspace, sends '^?'; the same happens with Ctrl+Backspace
• With the patch:
  • pressing Ctrl+V then Backspace sends '^?'
  • pressing Ctrl+V then Ctrl+Backspace sends '^H'

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: ngraham

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D10884