Moved SeparatorWidget into widgets/

It's now abstracted by the FrameworkWidgetFactory, allowing
the user to provide his own

src/FrameworkWidgetFactory.cpp

@@ -24,6 +24,7 @@
 #include "widgets/TitleBarWidget_p.h"
 #include "widgets/TabBarWidget_p.h"
 #include "widgets/TabWidgetWidget_p.h"
+#include "widgets/SeparatorWidget_p.h"
 
 using namespace KDDockWidgets;
 
@@ -55,3 +56,8 @@ TabWidget *DefaultWidgetFactory::createTabWidget(QWidget *parent) const
 {
     return new TabWidgetWidget(parent);
 }
+
+SeparatorWidget *DefaultWidgetFactory::createSeparator(Anchor *anchor, QWidget *parent) const
+{
+    return new SeparatorWidget(anchor, parent);
+}