Lossless optimisation of PNG image files in clipart Gallery.
Total saving of 887 KB !
Used ImageOptim.com and custom tools ('pngslim').

- Arrows: Saved ~44KB out of 189KB. ~23% per file on average (up to 38%).
- Computers: Saved ~17KB out of 242KB. ~7% per file on average (up to 11%).
- Education: Saved ~142KB out of 804KB. ~18% per file on average (up to 36%).
- Environment: Saved ~197KB out of 1.3MB. ~14% per file on average (up to 23%).
- Finance: Saved ~67KB out of 695KB. ~10% per file on average (up to 37%).
- People: Saved ~280KB out of 2.7MB. ~10% per file on average (up to 15%).
- Transportation: Saved ~141KB out of 1MB. ~14% per file on average (up to 15%).

Change-Id: I1812ae84e5672d49a7798dc44f518c1d6cd70ec1
Note: These should ultimately be replaced with original (/improved) SVG vector artwork.
Signed-off-by: Andrew <dent.ace@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/15669Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>
Tested-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>