Add web resource text source option

A few problems are noted in the added code in TextOutputSource.cpp, first of all, the bounding box size is not properly updated when the text is updated. Secondly, if the user hides or deletes a scene text item just after the web request has been sent, the whole scene has to wait for it. I was not able to find a direct workaround/fix for this. This is especially not favorable as it also affects the video output.

Once these problems are hammered out in one way or another, JSON/XML deserializing for this would be nice.

Text fading functionality like in image slideshow when the text changes could maybe be something to work on as well?