The individual callers should not be responsible for padding

This change handles message parse errors by printing the error when the
user tries to view the message. Specifically only handling unknown
charset errors in this patch, but there are many types of invalid
messages that can be handled in this way.

aerc currently leaves certain messages in the msglist in the pending
(spinner) state, and I'm unable to view or modify the message. aerc also
only prints parse errors with message when they are initially loaded.
This UX is a little better, because you can still see the header info
about the message, and if you try to view it, you will see the specific
error.

The `:rmdir` command removes the current directory (`-f` is required if
the directory is not empty).

This is not supported on the notmuch backend.

An issue with the maildir backend is that some sync programs (e.g.
offlineimap) may recover the directory after it is deleted.  They need
to specifically be configured to accept deletions, or special commands
need to be executed (e.g. `offlineimap --delete-folder`) to properly
delete folders.

A danger of using this on the IMAP backend is that it is possible for a
new message to be added to the directory and for aerc to not show it
immediately (due to a slow connection) - using `:rmdir` at this moment
(with `-f` if the directory already contains messages) would delete the
directory and the new message that just arrived (and all other
contents).  This is documented in aerc(1) so that users are aware of
possible risks.

This changes the scrolling to be done on the draw, when the height is
updated, ensuring that the selected item is kept on screen during
resizing.

Also, this ensures that messages will fill the screen when resizing the
window, for instance, shrinking and then growing drags down more
messages if possible.

This is a transplant of the dirlist scrolling logic.

This reverts commit 0f78f06610c0e8887aba2ae50e99b86477a384b3.

This reverts commit f06d683688e3d2139b14f67b7e349089e7200bf4.

We always set 10 seconds anyhow, might as well do that without repeating ourselfs.

The following functionalities are added to configure aerc ui styles.
- Read stylesets from file with very basic fnmatch wildcard matching
- Add default styleset
- Support different stylesets as part of UiConfig allowing contextual
  styles.
- Move widgets/ui elements to use the stylesets.
- Add configuration manual for the styleset