Just use the full filename instead, this is simpler and more explicit.

The former fixes encoding issues in filenames. The latter is just for
brevity.

References: https://todo.sr.ht/~sircmpwn/koushin/1

This allows to go back and forth between the mailbox view and the
message view.

References: https://todo.sr.ht/~sircmpwn/koushin/22

References: https://todo.sr.ht/~sircmpwn/koushin/22

This fixes the second part of #14.

References: https://todo.sr.ht/~sircmpwn/koushin/14

References: https://todo.sr.ht/~sircmpwn/koushin/19

This adds support for additional charsets when parsing e.g. subject
lines.

Closes: https://todo.sr.ht/~sircmpwn/koushin/14

HTTP requests can be processed in parallel, but we only have one IMAP
connection per session.

Closes: https://todo.sr.ht/~sircmpwn/koushin/12

Enables support for non-UTF8 charsets.

Closes: https://todo.sr.ht/~sircmpwn/koushin/18

This is required for authenticating with the SMTP server when composing
a new message.