[MlMt] keybinding to load external references

[Jonathan Clark]

>> "^L" = {"loadImagesOnce:"};  // ctrl-shift-l

> I use the following in my custom keybindings file:
>
> "L" = "loadImagesOnce:"; // load external references (e.g. images) 
> once

Aha! I had braces, which I now realise must indicate starting a list. 
Which confused things.

> The uppercase L is activated on the keyboard with Shift+L, just as 
> when typing an uppercase L. It works just fine.

And on my other point 1, I confirm MailMate needs to re-start before it 
picked up this change to my custom keybinding plist.

Thanks for your help.

-- Jonathan