Optimize PNG files

[dilyanpalauzov]

by calling for i in $(find -name '*.png'); do optipng -o7 -zm9 $i; done.

[what-the-diff]

• The image files in the docs/guide/images folder were changed.
• The images for jquery ui theme smoothness was updated to a newer version of jQuery UI (1.12).

[samdark]

Thanks.

[marcovtwout]

@dilyanpalauzov Could you tell me why we would not use the officially distributed files from jQueryUI and instead optimize these?

[samdark]

@marcovtwout just these are a bit smaller :) I did that in the past but it was lost during one of the updates.

[marcovtwout]

@samdark Thanks for clearning that up. One might otherwise wonder why vendor files are modified / will give a binary signature mismatch. To save just a few hundred bytes I would personally use the originals, but if this was common practice before in yii1 I guess it makes sense.

[dilyanpalauzov]

I optimized the PNG files in Limesurvey. Then I was told that once upstream synchronization happens, the changes will be reverted. In turn I optimized the PNG files used upstream projects, like here or jquery/jquery-ui#2132.