Re-ordering text elements in the slide DOM to match their visual order

[banksJeremy]

I think that SVG elements are just exported from LibreOffice in their internal document order. A text box may appear to be at the top of the page, but if it was added later than others it may be the end of the document. This produce a very confusing result when trying to select text.

We should probably re-order the text elements in the slides' DOMs to correct this.