libgimpwidgets: improve/fix more of GimpMemSizeEntry.

Looking further at this widget, many things are not right. Here are the
changes:
- Use binary prefixes (i.e. kibibyte, mebibyte and gibibyte) instead of
  decimal ones. We are making binary shifts so we were actually showing
  the wrong units.
- Round the value to the closest integer when showing it, not towards 0.
  Otherwise I had cases where it was showing 7GiB for an actual value of
  7.69GiB (default as computed by GIMP from my actual physical memory).
  Note that I am actually unsure even rounding makes sense. Shouldn't we
  rather show double values with a few digits after the decimal points?
  For such values, I think it would make sense.
- Do not edit the internally saved accurate value when the entry is
  edited to the same less accurate value as our saved value would be
  shown too. In particular when changing the display unit to a bigger
  one, we don't want to lose accuracy. This is especially true for low
  values. Say you don't have a lot of memory and you set the Tile cache
  size to 1.5GiB (1536MiB), you certainly don't want it to become either
  1 or 2GiB when switching display unit to GiB. Now even if the number
  will still display with less accuracy, the internal value will stay
  accurate.

(cherry picked from commit d886bb1b)
3 jobs for gimp-2-10 in 74 minutes and 8 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Dependencies
passed #842488
deps-debian/testing

00:14:06

 
  Gimp
passed #842489
gimp-debian/testing-autotools

00:30:52

failed #842490
allowed to fail
gimp-debian/testing-autotools-distcheck

01:00:01

 
Name Stage Failure
failed
gimp-debian/testing-autotools-distcheck Gimp The script exceeded the maximum execution time set for the job
Making installcheck in imagemap
make[3]: Entering directory '/builds/GNOME/gimp/_build/gimp-2.10.21/_build/sub/plug-ins/imagemap'
Making installcheck in images
make[4]: Entering directory '/builds/GNOME/gimp/_build/gimp-2.10.21/_build/sub/plug-ins/imagemap/images'
make[4]: Nothing to be done for 'installcheck'.
make[4]: Leaving directory '/builds/GNOME/gimp/_build/gimp-2.10.21/_build/sub/plug-ins/imagemap/images'
Uploading artifacts for failed job
Pulling docker image gitlab/gitlab-runner-helper:x86_64-6fbc7474 ...
ERROR: Job failed: execution took longer than 1h0m0s seconds