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
Depends on !4798 (merged).
Marking as draft since I am waiting for more feedback on #190 (closed).