Update netclasses when loading footprint

When clearing the pcb in footprint editor, we need to transfer the
netclass settings to ensure we have the correct defaults for the new
footprint.

Fixes: lp:1751583
* https://bugs.launchpad.net/kicad/+bug/1751583

pcbnew/initpcb.cpp

@@ -113,6 +113,7 @@ bool FOOTPRINT_EDIT_FRAME::Clear_Pcb( bool aQuery )
     if( GetBoard() )
         board->SetDesignSettings( GetBoard()->GetDesignSettings() );
 
+    board->SynchronizeNetsAndNetClasses();
     SetBoard( board );
 
     SetCurItem( NULL );