Avoid invoking internal functions in the demo programs

[jserv]

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.

[Damien-Chen]

All file in /src directory can use functions declared in twin_private.h, but all source file in /apps are not allowed to use it. Is my understanding correct ?

[jserv]

All file in /src directory can use functions declared in twin_private.h, but all source file in /apps are not allowed to use it. Is my understanding correct ?

Yes, it is intentional.