Optimize all png images using zopflipng (openhab#1555)

The command
```
for i in $(find . -name '*.png'); do zopflipng -m --filters=0me --lossy_8bit --lossy_transparent -y "$i" "$i"; done
```
was used to accomplish this optimization, all images should have lost no
accuracy as a result of the compression applied.

Signed-off-by: Ethan Dye <[email protected]>

Author: ecdye