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

src/kcms/tabbox/shortcutsettings.cpp

@@ -130,7 +130,7 @@ ShortcutSettings::ShortcutSettings(QObject *parent)
 
     // TabboxType::Main
     addShortcut(ki18nd("kwin", "Walk Through Windows"), Qt::ALT | Qt::Key_Tab);
-    addShortcut(ki18nd("kwin", "Walk Through Windows (Reverse)"), Qt::ALT | Qt::SHIFT | Qt::Key_Backtab);
+    addShortcut(ki18nd("kwin", "Walk Through Windows (Reverse)"), Qt::ALT | Qt::SHIFT | Qt::Key_Tab);
     addShortcut(ki18nd("kwin", "Walk Through Windows of Current Application"), Qt::ALT | Qt::Key_QuoteLeft);
     addShortcut(ki18nd("kwin", "Walk Through Windows of Current Application (Reverse)"), Qt::ALT | Qt::Key_AsciiTilde);
     // TabboxType::Alternative

src/tabbox/tabbox.cpp

@@ -359,7 +359,7 @@ static constexpr const auto s_appAltRev = kli18n("Walk Through Windows of Curren
 void TabBox::initShortcuts()
 {
     key(s_windows, &TabBox::slotWalkThroughWindows, Qt::ALT | Qt::Key_Tab);
-    key(s_windowsRev, &TabBox::slotWalkBackThroughWindows, Qt::ALT | Qt::SHIFT | Qt::Key_Backtab);
+    key(s_windowsRev, &TabBox::slotWalkBackThroughWindows, Qt::ALT | Qt::SHIFT | Qt::Key_Tab);
     key(s_app, &TabBox::slotWalkThroughCurrentAppWindows, Qt::ALT | Qt::Key_QuoteLeft);
     key(s_appRev, &TabBox::slotWalkBackThroughCurrentAppWindows, Qt::ALT | Qt::Key_AsciiTilde);
     key(s_windowsAlt, &TabBox::slotWalkThroughWindowsAlternative);