Create unallocated space within new extended partition after aligning boundaries (#757671)

When creating a new extended partition, the unallocated space within it
was being created before adjusting the partition boundaries for
alignment reasons.  This must be wrong.  Move creation of the
unallocated space to after adjusting the partition boundaries for
alignment reasons.  First introduced by this commit:

    a30f991c
    Fix size reduced by 1 MiB when created after cylinder aligned partition

Also added a big FIXME comment explaining how further adjustments are
still made by snap_to_alignment() to the partition boundaries including
a test case where this too late adjustment causes overlapping boundaries
and apply to fail.

Bug 757671 - Rework Dialog_Partition_New::Get_New_Partition() a bit