Don't use mixed units in size-column of details-view

This makes it tricky to compare the filesizes without adjusting
the sort-order, so now all sizes in the size-column are shown
in KiB or KB (dependent on the KLocale setting).

BUG: 219932
FIXED-IN: 4.8.0

Related fixes:
- Stay consistent with the rounding when using the KiB/KB unit
  in the statusbar.
- Fix sorting-by-size issue for folders
- Show "Unknown" in the size-column when the number of items
  cannot be determined.