[AvsFunctionDialog] Make 'define sliders' always appear in the same spot
Add 'toggle preview placement' menu option

In addition to at the bottom, the video preview can be placed now to
the right (when not using a separate window).  Default is bottom, but
the current placement is also stored when saving a session and will
be set when loading the session file if it's not empty, including the
auto-saved startup session.

Closes #22