view: add infrastructure to iterate through cursor columns

The number of columns i.e. maximal number of cursors located on the
same line can be obtained by view_cursors_column_count.

Column addressing is zero based, valid indexes are [0, max-1].

Assuming there is a cursor on every letter:

 a
 b c
 d e f
 g h
 i

max column would be 3, and the following would iterate over the
cursors forming the second column [c, e, h]:

 for (Cursor *c = view_cursors_column(view, 1); c; c = view_cursors_column_next(c, 1))
   ...

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