Make the board setup window quasimodal

By making it quasimodal, it allows the main KiCad window and Eeschema
to be used still, if it were just modal then it blocks out all other
windows.

Fixes https://gitlab.com/kicad/code/kicad/issues/4023

pcbnew/pcb_edit_frame.cpp

@@ -605,7 +605,7 @@ void PCB_EDIT_FRAME::DoShowBoardSetupDialog( const wxString& aInitialPage,
     if( !aInitialPage.IsEmpty() )
         dlg.SetInitialPage( aInitialPage, aInitialParentPage );
 
-    if( dlg.ShowModal() == wxID_OK )
+    if( dlg.ShowQuasiModal() == wxID_OK )
     {
         SaveProjectSettings();