Follow Plasma's virtual keyboard setting

KWin activates the input method for every text field; its policy (the
tray toggle) only gates the input panel, which sloposk doesn't use. sloposk
now asks KWin over D-Bus whether a focused field should show the
keyboard: never when off, only after touch or a pen by default.
"Always Show" and D-Bus Show/Toggle still show it.

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

tests/tst_visibility.cpp

@@ -16,6 +16,19 @@ private Q_SLOTS:
         QVERIFY(!v.isVisible());
     }
 
+    void fieldShowsOnlyWhenTheSystemWantsIt()
+    {
+        VisibilityController v;
+        v.setShowOnField(false);
+        v.setFieldActive(true);
+        QVERIFY(!v.isVisible());
+        v.show(); // explicit requests still work
+        QVERIFY(v.isVisible());
+        v.setShowOnField(true);
+        v.setFieldActive(true);
+        QVERIFY(v.isVisible());
+    }
+
     void dismissLastsUntilFocusChanges()
     {
         VisibilityController v;