Replace GTK_STOCK_OK with "_OK"

GTK 3.10 deprecates stock images, see
 https://docs.google.com/document/d/1KCVPoYQBqMbDP11tHPpjW6uaEHrvLUmcDPqKAppCY8o/pub

The developer documentation suggests to replace it with the label "_OK"
 https://developer.gnome.org/gtk3/3.9/gtk3-Stock-Items.html#GTK-STOCK-OK:CAPS
which is exactly what I do with this commit.

This fixes bug #8