Icon redux 01: Optimised help images

Help images (icon-themes/galaxy/res/helpimg) get repackaged into every icon theme shipped with LibO, so any reduction has a 5x impact on instal size.
- Remove localised help images that duplicate the standard set (i.e. they are not really locale specific).
- Optimised all remaining png files, to reduce file size with no visual impact.

QA - For viewing localised help images the following help topics are useful:
- Text Formatting Bar
- Line and Filling Bar
However, currently this function isn't working with online help (bugs fdo#75636 and fdo#75637).

Change-Id: If0e7525dfb7d2dd5455652684b86899ed5d9a5ab
Signed-off-by: Andrew <dent.ace@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/8404
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>