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