conversations: remember last seen timestamps

[aviau]

Or user won't see the message.

[aviau]

@isra17 review plz

[aviau]

ping @isra17!

After our discussion, I have implemented persistent timestamps.

It looks like this in the buddylist:

            <chat proto='prpl-flare-icq' account='+censored'>
                <alias>Salut</alias>
                <component name='group'>Conferences</component>
                <component name='title'>Salut</component>
                <component name='state'>joined</component>
                <component name='sn'>censored@chat.agent</component>
                <setting name='gtk-mute-sound' type='bool'>0</setting>
                <setting name='last_seen_timestamp' type='string'>1576258436</setting>
            </chat>

I use strings for the timestamp because they are int64.

[isra17]

LGTM!