Idea:

0.  Instead of a single mainwid, allow a grid of them starting from the normal
    1 x 1 setting up to 3 x 2, where the first number is the row count, and the
    second number is the column count.

1.  Each mainwid is associated with a set, starting from set 1 through set 32.

    1.  Each mainwid has its own set-spinner.  Just like the original single
        set spinner, this will alter the m_screenset member.

    2.  The following arrays are large (sizeof long x c_max_sequence),
        so duplicating them will add an extra 4K per array.

    3.  c_mainwnd_rows and c_mainwnd_cols (and the corresponding mainwid
        members) will remain at 4 and 8, respectively.

    4.  m_mainwid_x and _y will depend on the mainwid position.


x.  Operations like move will probably start out restricted to be within the
    same mainwid.

y.  Feature macro:  SEQ64_MULTI_MAINWID
