gdb/tui: add window width information to 'info win' output

gdb/tui: add window width information to 'info win' output

Now that we support horizontal window placement in the tui, it makes
sense to have 'info win' include the width, as well as the height, of
the currently visible windows.

That's what this commit does.  Example output is now:

  (gdb) info win
  Name       Lines Columns Focus
  src           12      40 (has focus)
  asm           12      41
  status         1      80
  cmd           11      80

I've added a NEWS entry, but the documentation was already suitably
vague, it just says that 'info win' displays the size of the visible
windows, so I don't think anything needs to be added there.

I've also added some tests, as far as I could find, the 'info win'
command was previously untested.