gtk3: Splitter idnores design settings and always 46pix width

Original Reporter info from Mantis: AntonK

Reporter name: Anton Kavalenka

Description:

Run the attached demo

Mantis conversion info:

Mantis ID: 34106
OS: Linux
OS Build: Debian 9.x
Platform: x86_64
Version: 1.9 (SVN)
Fixed in revision: r59339 (#f1335e9e), r59341 (#1d759559)