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

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

Change-Id: I1748e877a7958ba93635f2f6c2b0c88309a60d24
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.common/+/190439
Tested-by: Alexander Kurtakov <akurtako@redhat.com>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>