actions: Fix SwitchScreen screen value range Currently we accept values of -255..255 while parsing the screen value of `SwitchScreen` actions`, but then we silently cast it against `int8_t`, i.e. in range -128..127. We actually do as xkbcomp, but this seems a bug because the target storage is a `char`. Let’s simply raise a parse error if the value does not fit in our type.