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>

tests/tst_layoutbuilder.cpp

@@ -137,6 +137,19 @@ private Q_SLOTS:
         QCOMPARE(letterKey(sk, u"l"_s).alternates.first(), u"ľ");
     }
 
+    void fullModeLettersFlickToTheirAltGrSymbol()
+    {
+        const Layout sk = TestLayouts::build("sk", KeyboardMode::Full);
+        const KeyDef j = letterKey(sk, u"j"_s);
+        QCOMPARE(j.altGr, u"'");
+        QCOMPARE(j.flick, u"'");
+        QCOMPARE(j.text({ShiftLevel::None, true}), u"'");
+        QCOMPARE(letterKey(sk, u"ô"_s).flick, u"\""); // keys with a shifted symbol keep it
+        const KeyDef plus = letterKey(sk, u"+"_s);
+        QCOMPARE(plus.deadAt({ShiftLevel::Shifted, true}), quint32(XKB_KEY_dead_tilde));
+        QCOMPARE(plus.text({ShiftLevel::None, true}), u"!");
+    }
+
     void noPlaceholdersRemain()
     {
         for (const Layer &layer : TestLayouts::build("us").layers) {