Change keymap from ctrl-o to ctrl-alt-s

[machitgarha]

By default, ctrl-o keybinding is reserved for opening a file, replacing it is not a good idea: The user expects the file selection dialog to come up. A better alternative could be ctrl-alt-s, which also includes the first letter of the word "structure".

[CLAassistant]

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[machitgarha]

Sorry, but I cannot sign the CLA, simply because the access to cla-assistors.io is restricted from here (Iran). As it is a small change, either accept it or create another pull request with the same change. Thanks.