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>

Sometimes I observe out-of-order messages when using a maildir inbox. It
appears that the UIDs for these messages are returned out of order by
the MessageStore. In order for a maildir MessageStore to return messages
in most recently received order, it must have already opened all
messages and parsed the date to use as a sort key. Rather than implement
that, simply sort messages by time as we display. This fix shows my
emails in order.

Choose the readline defaults for the behavior of these two
functions/keybindings. Depending on the program, either of these can
delete the whole line.

Note that by default in [compose], <C-k> is bound to :prev-field<Enter>.
Leave it up to the user whether or not they want to rebind the key in
[compose].

This fixes ~sircmpwn/aerc2#182

https://todo.sr.ht/~sircmpwn/aerc2/182

Separatiing the ui code from aerc makes it usable as a library in
other projects.