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It's in the editor view, beside the undo button, at the top-right.

It's not visible if no changes have been made (if the consensus is that's a bad idea I'm happy to change it).

If no changes have been made since the last save you could replace the save button with a message that states something to that effect.

That sounds about right. Might fade it out and have that as the tooltip.

Disabled button is preferable to hiding