Remove special cases for rows==1 which assumed the "yes/no" layout
specifically.  This is cleanup for removing the "dynamic columns"
layout style, which is unused and complicates the menu internals.

Use 2 columns with dynamic rows for yes/no instead, which is exactly
the same since it always has two buttons.

Provide cairo_menu_item_rows() to get the actual number of rows in a
menu based on the current items.

Use cairo_menu_item_rows() to compute the frame and menu height
instead of assuming either 1 row or 'nitems' rows.

Size button widths based on the actual number of columns when the
columns are fixed.  This would change behavior for dynamic-column
layouts or layouts with >2 columns, but such a layout is not possible.
This pivots to a test for fixed vs. dynamic columns which faciliates
removing the dynamic columns layout.

Signed-off-by: Jonathon Hall <jonathon.hall@puri.sm>