Use regular "text" shapes to allow editing

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Regular text shapes are also drawn using an SVG that is independent of the actual text.
The text attribute could hold the label text and also the settings used to create it (e.g. as json string).
Assigning a fake font face prevents EasyEDA from trying to redraw the shape, so the actual SVG should be preserved.
This would allow future editing of labels within the extension.

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One issue though: https://easyeda.com/forum/topic/Changing-height-of-text-with-unknown-font-redraws-it-without-asking-0e2a7e72ce3443aca623996025d49401
Changing the height of a text redraws it with the default font immediately...