Print Studio: faster layout and more reliable re-setting

An ad from a chosen image suggestion and re-setting after a text change finish much faster. The image check is not repeated as long as the image stays the same, and the remaining checks run at the same time. Many quick changes in a row no longer pile up: the running layout finishes, followed by exactly one run with the latest content. If the rendering aborts, it is retried automatically, and a remaining problem is shown in plain text at the ad. Filling in from a description no longer breaks off for long texts.