Add a simple Scots Gaelic keymap.

There doesn't seem to be a standard layout, so this is modelled after the Irish Gaelic keymap: it's essentially a UK ISO layout (with double-quote on shift-2) but with AltGr+vowels producing the appropriate accents. Irish Gaelic and Scots Gaelic are considerably different, with the former using acute accents throughout and the latter using grave accents. As the layout is a strict superset of the standard UK English layout I've marked it as being applicable to both English and Gaelic.