Your suggestion fixed indeed the icon problem. Strange how but......it works and it reduces my errors significantly.
I will see tomorrow when the tarball with your fix is generated if this fix more problems. BTW I don't see that your fix is comitted in the git repository. My build still crashes when I resize the window. I believe in the past this was a problem with my toolchain. I wonder why it is back if I build from the Gtk3 branch and works correct for the master branch.
It seems the look is the same as you have. I don't like the way "Datenansicht" and "Variablenansicht" is displayed. But if this is the way it is....