Readd yield call to give slice to system.

Note:
- wxYield does not work here, indicating the need for locking
- wxYieldIfNeeded does not work, possibly for the same reason as wxYield
- Both Layout() and m_fieldGrid->Layout() need to be called even though
  the first should call the second.

Fixes https://gitlab.com/kicad/code/kicad/issues/5690

eeschema/dialogs/dialog_symbol_properties.cpp

@@ -518,6 +518,8 @@ bool DIALOG_SYMBOL_PROPERTIES::TransferDataToWindow()
     m_tcLibraryID->SetValue( UnescapeString( m_symbol->GetLibId().Format() ) );
 
     Layout();
+    m_fieldsGrid->Layout();
+    wxSafeYield();
 
     return true;
 }