Add Platform::focusedView() and respective signal

Implemented as QGuiApplication::focusObjectChanged() for Qt,
other frontends will have their own impl.

src/qtquick/Platform_qtquick.h

@@ -23,6 +23,7 @@ public:
     Platform_qtquick();
     ~Platform_qtquick() override;
     const char *name() const override;
+    std::shared_ptr<ViewWrapper> focusedView() const override;
 };
 
 }