Avoid invoking internal functions in the demo programs #9
Description
Some of the demo applications are using internal functions such as _twin_widget_init, which should never be visible to Mado users. Sooner or later, the header twin_private.h should be moved from the include directory to the src directory, where the implementation parts are located. Known applications that should be changed include apps/clock.c, apps/line.c, and apps/spline.c.