Two variants:
* Czech Standard (= qwertz)
* Czech qwerty
The accented letters layout corresponds to the UCW [1] layout shipped
with xkb, so we follow this precedent.
Like the DE layout, and unlike the US layout, the CZ layout has a
comma key and a narrower space bar. The added comma key also serves as
a visual balance to the added accents key, to keep the spacebar
centered.
The layouts have been tested manually on PinePhone.
[1] c60b77ea51/test/data/symbols/cz (L180)
c3b428e517