Bug 521926: Eclipse Doc bundles are quite huge (II)

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

Change-Id: Iac8cd43d0b9c0371d98af94de2ba0b5b422dd82f
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.common/+/190437
Tested-by: Alexander Kurtakov <akurtako@redhat.com>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>