Signed-off-by: Wagner Riffel <wgrriffel@gmail.com>

This adds the ability for per-account signatures in the accounts.conf
config file. The signature is added to emails in the editor at the
bottom of the email. This includes when forwarding, replying to, and
composing emails.

There are two config options: signature-file and signature-cmd. The
former allows a signature to be read from a file and the latter allows
an arbitrary command to be executed to return the signature.

The config options have been documented in aerc-config

This adds the event type as well as the command implementation, but no backend
supports it yet.

Add an onUpdateDirs handler. This is used to invalidate the dirlist and
redraw with the correct number of recent/unread/total messages is shown.

A config option and formatting options are provided.

This adds the Mouseable interface. When this is implemented for a
component that item can accept and process mouseevents.

At the top level when a mouse event is received it is passed to the
grid's handler and then it trickles down until it reaches a component
that can actually handle it, such as the tablist, dirlist or msglist.

A mouse event is passed so that components can handle other things such
as scrolling with the mousewheel. The components themselves then perform
the necessary actions.

Clicking emails in the messagelist opens them in a new tab.

Textinputs can be clicked to position the cursor inside them.

Mouseevents are not forwarded to the terminal at the moment.

Elements which do not handle mouse events are not required to implement
the Mouseable interface.

Signed-off-by: Wagner Riffel <wgrriffel@gmail.com>

Os.SET_SEEK is deprecated, it's recommended to use contants from io
package

Signed-off-by: Wagner Riffel <wgrriffel@gmail.com>

Signed-off-by: Wagner Riffel <wgrriffel@gmail.com>

With sort.Sort, sometimes two messages with the same timestamp will switch
places every time the message list is redrawn