add support for standard ctrl/cmd-s shortcut

[StevenClontz]

WIP

[kenmonks]

Thanks for submitting this.

Initially the hotkey for save was set to the standard CTRL/CMD+s. But I intentionally changed it to ALT/OPTION+s because the browser itself uses CTRL/CMD+s for its File>Save menu item, and that opens the same OS system file saving dialog that Lurch does, so it's very hard to disambiguate which dialog you have opened. The browser's Save menu option is saving the web page containing Lurch, but the Lurch Save menu is saving the Lurch document in the editor.

Which Save menu the hotkey activates if they are both set to CTRL/CMD+s depends on whether or not the editor currently has focus. That isn't always obvious unless the user intentionally checks it before pressing the hotkey by clicking inside the editor. The bottom line is that if the hotkeys for saving are the same, it is extremely easy for a user to press the hotkey, save the wrong thing, and lose their work.

We could add an option to allow the user to customize their own hotkeys for the menu at some point, then they can use it at their own risk. But for student using the app to do assignments, the default probably should be to avoid the conflict with the browser's save menu hotkey for now.