ComposeKey: Add option for wider spacebar

[djbclark]

On 10" tablets (e.g. Acer Chrometab 10) in portrait mode, for people with small hands it'd be great to have an option for a wider and/or repositioned more to one side and/or split with seldom used keys in the very center spacebar.

I'm annoyed enough by this that I'm going to see if I can figure out how to do this, but any quick hints from clueful people on how to properly implement this so changes might get merged in would be welcomed.

Specifically would it be better/possible to make this a global option for every keyboard type, or better/required by API limitations to add a new selectable via "Add input methods" keyboard as a slight, perhaps programmatically created variant of each existing ComposeKey keyboard layout.