This first step only affects GTK3, later we will extend the support
to other platforms.

Note that these images are derived from the OSX PNG files, not the header-file
encoded data we currently use for gtk/gtk3.

Also rename the files to more useful names.

Change-Id: Ia13a3f2ac35b06672aff724f4cf5bdcd823f6342
Reviewed-on: https://gerrit.libreoffice.org/67528
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>