Headers: order flags according to mu4e-headers-visible-flags

[andersjohansson]

This is more of a question, but I tried out an implementation Looking at flags after #1792, I figured that we would want them to be displayed in a specified order, and this was also what the docstring of mu4e-headers-visible-flags seemed to imply. Was this intended from the beginning?

This of course breaks default expectations. Probably mu4e-headers-visible-flags would need to be reordered for keeping the expected order.