Split most widgets into a Controller and a View

Controllers don't have any GUI and don't depend on the frontend
technology.

Views are implemented in a specific GUI tech (QtWidgets, QtQuick, etc).

For now only QtWidgets work. There's still a lot to decouple.

This will make it easier to introduce non-Qt backends.

src/fwd_headers/kddockwidgets/KDDockWidgets.h

@@ -10,3 +10,11 @@
 */
 
 #include "../../KDDockWidgets.h"
+
+class QWidget;
+
+namespace KDDockWidgets {
+// TODO remove
+using QWidgetOrQuick = QWidget;
+using WidgetType = QWidget;
+}