kcms/tabbox: change Shift+Backtab to Shift+Tab

In default shortcuts, change +Shift+Backtab to +Shift+Tab.

Functionally the behavior doesn't change.
But Shift+Tab is better since it is
- easier to understand (not everyone knows what Backtab is)
- more consistent with other shortcuts containing Shift:
  we use Alt+Shift+1, instead of Alt+Shift+!
- more consistent with how user defined shortcuts with Shift and Tab are
  displayed and stored

BUG: 422713
FIXED-IN: 6.0