pcbnew: Reload board after changing board paramters

The old board pointer maintains shared pointers to netclasses (among
other things) that do not get released by the
SynchronizeNetsAndClasses() call.  We reload the board to fully release
the pointers.

Fixes: lp:1835533
* https://bugs.launchpad.net/kicad/+bug/1835533

pcbnew/dialogs/panel_setup_netclasses.cpp

@@ -267,6 +267,7 @@ bool PANEL_SETUP_NETCLASSES::TransferDataFromWindow()
 
     m_Pcb->SynchronizeNetsAndNetClasses();
     m_BrdSettings->SetCurrentNetClass( NETCLASS::Default );
+    m_Frame->SetBoard( m_Pcb );
 
     return true;
 }