Add an AltGr key; full-mode letters flick to their AltGr symbol

The right alt in full mode is now AltGr: sticky like the other modifiers,
it types the keymap's third/fourth level (sk: AltGr+J = ') instead of an
Alt shortcut. In hold-keys mode it holds the physical right alt, since
KWin resets the modifier state after an ISO_Level3_Shift keysym.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/layout/KeymapKeys.h

@@ -5,8 +5,8 @@
 
 struct xkb_keymap;
 
-// One typing key of a keymap layout, at its two shift levels. A level is
-// either text or a dead key (then `text` is its spacing accent, for labels).
+// One typing key of a keymap layout, at its shift and AltGr levels. A level
+// is either text or a dead key (then the text is its spacing accent, for labels).
 struct KeymapKey {
     enum class Position { Regular, Backslash, LessGreater }; // BKSL, LSGT: not on every keyboard
 
@@ -15,6 +15,10 @@ struct KeymapKey {
     QString upper;
     quint32 deadLower = 0; // dead keysym, 0 if the level types text
     quint32 deadUpper = 0;
+    QString altGr;      // third level (AltGr), empty if it types nothing
+    QString altGrUpper; // fourth level (AltGr+Shift)
+    quint32 deadAltGr = 0;
+    quint32 deadAltGrUpper = 0;
 
     bool isLetter() const;
 };