Feature of ScrollTextWidget, only used for now by TextEditor.
Pan is like Swipe, but wait a bit at end of gesture to release:
the line on which Pan was started will be moved to where Pan is
released.
May conflict with MovableContainer (so not enabled for DictQuickLookup,
where it could have been nice - but it would work only with text
dictionaries, not with HTML ones, as ScrollHtmlWidget can't really
do that).