vim: custom key rows specifiable via ?status

vim: custom key rows specifiable via ?status

Allows a non-interactive keyboard custom-tailored to show only wanted key
modes and cases.  Useful to print out specific rows.

Letters indicate Vim modes (e.g. g, ^w, v, i, nothing for normal), following
digits specify keyboard modes/cases (where 1 is lower case, 2 shifted, and
3 with control pressed).  After this, a dash with more digits can be added
to supply alternative key cases shown only at the top (numeric) row.

For example:  ?static=i3+z1+g1-2+321-21  would result in:

1. insert mode keys with control pressed;
2. z + unshifted keys;
3. g + shifted keys for the first row, so !-) instead of 1-0,
     + normal keys (a-z, not A-Z as would be g2) elsewhere;
4. standard rows (control, shift, normal) for normal mode,
   but leaving out ctrl for the first row (only ^^ there).

Clicking mode keys will not work correctly (should probably just be
disabled).