Icon Redux 02: Optimised Sifr theme

Lossless optimisation of Sifr PNG files, primarily removing superfluous metadata (ImageOptim.com). Saved 303.6KB out of 555.6KB. 54.6% overall (up to 97% per file).

QA- Inspect icons with Sifr theme selected, using all aspects of the office suite. There should be no visible changes.

Change-Id: Ie5f1c73165d7b3373e7f10015eedb1d4a865ded9
Signed-off-by: Andrew <dent.ace@gmail.com>