Be aware that this PR is targeting message-sorting branch, not master branch.
Decided to make changes in separate branch to make it easier to see what would be changed.
@Athozus

Reworks mail.sort_messages(...) function to decouple it from user interface handles / formspec fields.

Function was used like this:

-- second argument sorting field, last argument is sort direction (both are formspec dropdown indexes)
local sortedmessages = mail.sort_messages(messages, "2", "2")

And after changes done here it will be used like this:

-- second argument sorting field, last argument is sort direction (ascending if true)
local sortedmessages = mail.sort_messages(messages, "subject", true)