When the mouse is enabled, clicking on a header field switches focus to
that field (likewise for the terminal).

Allowing the user to view deleted messages creates all sorts of race
conditions. The most devious race condition is pv.source can be set to a
nil while another PartViewer is still running a goroutine in
attemptCopy.

Here is a trace when this happens.

goroutine 76 [running]: io.copyBuffer(0x7f8ad02641d0, 0xc00040f590, 0x0,
0x0, 0xc0007cc000, 0x8000, 0x8000, 0x0, 0x0, 0x8b3d60)
/usr/lib/go/src/io/io.go:402 +0x101 io.Copy(...)
/usr/lib/go/src/io/io.go:364
git.sr.ht/~sircmpwn/aerc/widgets.(*PartViewer).attemptCopy.func4(0xc00017efd0,
0xc0004da7c0) /home/keur/repos/aerc/widgets/msgviewer.go:576 +0x611
created by git.sr.ht/~sircmpwn/aerc/widgets.(*PartViewer).attemptCopy
/home/keur/repos/aerc/widgets/msgviewer.go:544 +0x144

We could add a guard in store.FetchBodyPart to only call the callback
when msg.Part.Reader != nil, but we still get a hanging pager. Therefore
it seems more reasonable to disable this completely.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

%F now shows the auth name or recepient name/address if the
message is from you.

Adds a message indicating the user's ability to :save or :pipe a message
with an unsupported mimetype and also adds a selector widget (similar to
the tutorial).

The selector widget was previously defined in the account wizard module,
so this commit breaks it out into its own module to allow for re-use.

Further, modify the BeginExLine() function to take an argument that
pre-populates the command line, allowing functions to initiate an ex
command without executing it.

Closes #95.

When viewing a message part with no available filter, clicking the UI
would cause a nil pointer dereference because the MouseEvent was passed
to the PartViewer's `term` field, which does not exist in the case of an
absent filter.

When deleting a message, sometimes FetchDirectoryContents will fire.
FetchDirectoryContents will return a smaller set of UIDs since messages
have been deleted. This operation races with fetching from the seqMap in
client.ExpungeUpdate. Only recreate the seqMap if it can grow so that
messages will continue to be expunged.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>