[mlview-list][patch] Completion widget - UI fixes

[Nicolas Centa <nicolas centa free fr>]

Hello,

	With this patch :
- the completion widget is hidden when completion is OFF or when no DTD
is associated with the document
- the completion widget is disabled (unsensitive) when a node which is
not an element is selected
- the completion widget's scrollbars only appear when needed.

Cheers,

Nicolas Centa