Introduce ViewWrapper - a non-owning view

In many cases we need to access widgets that weren't created by
KDDW, in that case we have no View. Since all our APIs want to
deal in View and not in QWidget, we need a way to wrap stuff like
QWidget::window() and windowHandle()

All methods are unimplemented. Will only implement those that
are required/used.

src/views_qtwidgets/View_qtwidgets.h

@@ -14,6 +14,7 @@
 #include "private/Utils_p.h"
 #include "Controller.h"
 #include "View.h"
+#include "ViewWrapper_qtwidgets.h"
 
 #include <QDebug>
 #include <QEvent>
@@ -309,6 +310,12 @@ public:
         return QWidget::windowHandle();
     }
 
+    std::unique_ptr<ViewWrapper> window() const
+    {
+        ViewWrapper *wrapper = new ViewWrapper_qtwidgets(QWidget::window());
+        return std::unique_ptr<ViewWrapper>(wrapper);
+    }
+
 protected:
     bool event(QEvent *e) override
     {