Use WidgetScrollBox and WidgetLabel in MenuTalker

Closes #563
Fix MenuTalker's scrollbox stealing input

Also, WidgetScrollBox has been updated to support scrolling via keyboard
if the scrollbox doesn't contain any child widgets (i.e. plain text as
in MenuLog/MenuTalker).
Fix navigating WidgetScrollBox moving to next widget
Only allow one instance per Widget in a Tablist

Using this, we can easily add/remove Widgets as we toggle them in places
like MenuTalker. In MenuTalker, either the advance button or the close
button is show, so we don't want to allow selection of the disabled
button.
Use WidgetScrollBox as caption buffer in GameStateCutscene

Closes #556
Merge remote-tracking branch 'upstream/master' into interface
Remove hard-coded scroll amount in WidgetScrollBox
Merge pull request #650 from dorkster/interface

Use a WidgetLabel and a WidgetScrollBox for text buffering in MenuTalker