[PanelShadows] Update m_windows in setEnabledBorders

m_windows keeps track of which window has which borders so it can update them when e.g. theme changes.
Unfortunately, it was never updated after the window was initially added.

Differential Revision: https://phabricator.kde.org/D7505

shell/panelshadows.cpp

@@ -175,6 +175,7 @@ void PanelShadows::setEnabledBorders(const QWindow *window, Plasma::FrameSvg::En
         return;
     }
 
+    d->m_windows[window] = enabledBorders;
     d->updateShadow(window, enabledBorders);
 }