gdb/tui: fairer distribution of excess space during apply

gdb/tui: fairer distribution of excess space during apply

When applying layouts gdb computes the size of each window (or rather,
each sub-layout within a layout) using integer arithmetic.  As this
rounds down the results, then, when all sub-layouts are sized, there
is the possibility that we have some space left over.

Currently, this space is just assigned to an arbitrary sub-layout.

This can result in some unbalanced results.  Consider this set of
steps with current master:

  (gdb) tui enable
  (gdb) layout regs
  (gdb) info win
  Name       Lines Columns Focus
  regs           7      80
  src            9      80 (has focus)
  status         1      80
  cmd            8      80

Notice the weird split between the src and regs windows, the original
layout specification has these windows given equal weight.  The
problem is that, with rounding, both the regs and src windows are
initially sized to 7, the extra 2 lines are then arbitrarily added to
the src window.

In this commit, rather than add all the extra space to one single
window, I instead hand out the extra space 1 line at a time, looping
over all the sub-layouts.  We take care to respect the min/max sizes,
and so, we now get this result:

  (gdb) tui enable
  (gdb) layout regs
  (gdb) info win
  Name       Lines Columns Focus
  regs           8      80
  src            8      80 (has focus)
  status         1      80
  cmd            8      80

This looks more natural to me.

This is obviously a change in behaviour, and so, lots of the existing
tests need to be updated to take this into account.  None of the
changes are huge, it's just a line or two (or column for width) moved
between windows.