app: add a simple 12-hr clock

[ryzokuken]

/cc @gfwilliams wdyt?

[ryzokuken]

I wonder if a better option here would be to add a setting called "12-hour clock" and expect clock devs to handle that (then I'd remove the new app and just add a condition in the main sclock).

[gfwilliams]

Looks good - thanks!

I'm in two minds about the 24 hour thing - in stuff like this it makes a lot of sense, but then I'm also a big fan of keeping stuff as simple as possible.

I'll merge this now and I've filed an issue on #76