Lossless PNG optimisation for all Icon Themes.
Icons were run through ImageOptim 1.5.4 (https://imageoptim.com/). Total saving ~839KB.

Breeze: Saved 13.9KB out of 29.1KB. 47.5% per file on average (up to 54.5%).
Crystal: Saved 44KB out of 1.2MB. 3.7% overall (up to 85.8% per file).
Galaxy: Saved 10.7KB out of 2MB. 0.5% per file on average (up to 56%).
Hicontrast: A few bytes.
Human: Saved 288KB out of 1.8MB. 15.8% overall (up to 81.1% per file).
Industrial: Saved 22.3KB out of 1.1MB. 1.9% overall (up to 88.7% per file).
Oxygen: Saved 177.8KB out of 2MB. 9% overall (up to 95.3% per file).
Sifr: Saved 41.2KB out of 315.8KB. 13% overall (up to 63.4% per file).
Tango: Saved 130.8KB out of 1MB. 13.1% overall (up to 86.4% per file).
Tango_testing: Saved 110.3KB out of 546.7KB. 20.2% overall (up to 86.4% per file).

Change-Id: Id43faba59fa738cd2cde60df61c16d8cf2e06daf
Signed-off-by: Andrew <dent.ace@gmail.com>