Lossless optimization of PNG files in icon themes. Using ImageOptim (and some other tools). April 2015 update.
Sifr: Saved 14KB out of 293KB. ~5% overall (up to 60% per file).
Tango: Saved 10KB out of 749KB. ~1% per file on average (up to 53.1%).
Small improvements to other themes: Breeze, Galaxy, Human, Oxygen and Tango Testing.

Change-Id: Ib82ab16eaa3eb7a617e768a8b2fe7f04b43f9d7f
Signed-off-by: Andrew <dent.ace@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/15472Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>