Make DockWidgetBase::eventFilter() protected

The users might want to override it

Fixes #148 and restores source compat with v1.1

Changelog

@@ -5,6 +5,8 @@
   - Support for resizing dock widgets when they are in overlay/popup mode (autohide/sidebar feature)
   - Fixed title bar close button enabled state not being restored with Layout saver (#137)
   - Installs a version header (kddockwidgets_version.h) that defines a version string and other useful versioning macros (#138)
+  - DockWidgetBase::eventFilter() is protected instead of private (regression vs v1.1) (#148)
+    It's recommended that you rebuild your application when updating KDDW, as MSVC encodes private/protected in the name mangling.
 
 * v1.2.0 (17 December 2020)
   - Wayland support

src/DockWidgetBase.h

@@ -498,6 +498,9 @@ 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;
@@ -567,9 +570,6 @@ private:
     ///@brief Updates the floatAction state
     void updateFloatAction();
 
-    ///@reimp
-    bool eventFilter(QObject *, QEvent *) override;
-
     class Private;
     Private *const d;
 };