Alpar had the key, focus can be set in the window class. But it is not enough, the focused widget has to be activated, as well! Was a hard task to find out... By the way, two compilation warnings are removed.
2 Title: LEMON Reference Manual
3 Author: LEMON Development Team
4 Abstract: Documentation of LEMON.
8 Index: /usr/share/doc/lemon/html/index.html
9 Files: /usr/share/doc/lemon/html/*.html