[gta02-core] fped: new way of adding frame references

[Werner Almesberger]

As of SVN revision 5937, the frame icon in the toolbar (on the right)
is no more. With it, the contorted procedure for adding frame
references has disappeared.

The new way is much simpler: select the parent frame and then drag
the name of the child frame to the point of attachment. That's all.

I've also updated the documentation:
http://people.openmoko.org/werner/fped/gui.html

- Werner