Move DockWidgetBase::eventFilter() into the private class

Fixes issue #151 where the user is overriding eventFilter too
and getting into some edge cases.

Could be worked around in user code, but it's always good to have
less protected and private API in public classes.

src/DockWidgetBase.h

@@ -463,9 +463,6 @@ protected:
     void onShown(bool spontaneous);
     void onHidden(bool spontaneous);
 
-    ///@reimp
-    bool eventFilter(QObject *, QEvent *) override;
-
 #ifndef PYTHON_BINDINGS //Pyside bug: https://bugreports.qt.io/projects/PYSIDE/issues/PYSIDE-1327
     void onCloseEvent(QCloseEvent *e) override;
     bool onResize(QSize newSize) override;