Category colours and sorting.

Implement the final colours mentioned in #1058 (closed). As part of this, I also made the category chip show white or black text, based on how pale the colour in the background is. Without this the white text is difficult to read for some categories after you touch "View all".

While here, I've also sorted the "Top X apps" for each category shown in the category overview screen so that they show the most recently first. This prevents the overview screen from being exactly the same each time you view it. Right now the UX seems to be focused on the idea of discovering content, and it is silly to only ever discover the same, stale apps there.