Update lua and toolbar log controls

Reformat error message to current proposed "standard" and make it capable
of being internationalised.

Now return a Boolean on success or failure to start or stop logging and an
(internationaliseable) message indicating what happened, if anything and
including the log file-name if applicable.

In light of the messages now available to the lua command the code has been
revised so it will no longer cause a logging starting/stopping system
message to appear on the main console - though using the button on the main
console will continue to do so.)

The existing log-to-file button used an icon that appears to have been an
old Oxygen "folder-downloads.png" icon image and the only way to tell if it
was active was a system dependent colouration that "on" widgets adopt.  I
have taken the current 48x48 Oxygen "text-x-log.png" and superimposed
smaller (32x32) "media-record.png" and "media-stop.png" to create two
similar in style but visibly different "log-record" and "log-stop" icons,
they, like any other icons I have produced from the Oxygen icon set, are
released under a GPLv2+ licence for use in Mudlet and anywhere else that
is permitted.

Note that this commit/PR follows on from #208 and seems to have incorporated
it here as the first of the two commits - which _should_ be silently fast-forwarded
over when their both get merged. :sweat: 
The icons look really cool, but the record/stop buttons are like 3 pixels wide on the actual icon and are indistinguishable from each other, and you definitely can't read the 'log' text.

Perhaps try taking out the paper in the back and enlarging the circle to be the whole icon - that would look better on the small space we've got on the button.