Check how slides will look

Warnings about how the slides of a presentation look: a text that runs past its block or code that is cut off ("overflow"), a block partly or wholly outside the slide ("cut", "outside"), text blocks lying over each other ("overlap"). Texts and code are measured in a browser on the server ("measured": true); when that is not available their sizes are estimated from the letter widths of the font. Only what shows counts: a text on a block without fill or border may run into the bottom padding. The changing operations add the (estimated) warnings of the slides they changed to their answer ("layout_warnings").