Bug 521926: Eclipse Doc bundles are quite huge

Optimize the PNGs with "optipng -o7 -preserve -strip all"
This reduces file size but does not create visual differences.

Change-Id: I8fc1ed1dbda4c3040b70fe2d6a38ffd595bd1a28
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.common/+/190431
Tested-by: Alexander Kurtakov <akurtako@redhat.com>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>