RFC: in all default shortcuts, always use Shift+Tab instead of Shift+Backtab

Rationale

• For users, Shift+Tab is easier to understand than Shift+Backtab. The tab key is physically present on the computers, while some people may never have heard of Backtab before
• It is more consistent with other shortcuts containing shift -- we use Alt+Shift+1 instead of Alt+Shift+!
• It is more consistent with how we store and display the shortcuts when users assign it -- in settings, Shift+Tab is displayed and stored instead of Shift+Backtab.

Cons

• Some shortcuts might break.
  • Currently the code is designed to be robust to Shift+Tab/Backtab. The remaining issues should be solved by !4798 (merged).
• This would be a user facing change after beta 1.

Current shortcuts affected

• "kwin", "Walk Through Windows (Reverse)". Files to be changed:
  • kwin/src/tabbox/tabbox.cpp
  • kwin/src/kcms/tabbox/shortcutsettings.cpp
  • kglobalacceld/autotests/kglobalshortcutsrc.expected
  • kglobalacceld/autotests/kglobalshortcutsrc

Additional proposal

• make a kglobalshortcutsrc autotest that fails when it detects a default shortcut containing backtab

CCBUG: 422713

@teams/usability