ActReact.hh: new unit.
Stimulus: new wrapper.
Base.inl: remove Stimulus typedef; fix names of two field proxy macros.

HUD.hh: fully document unit.
CanvasPoint: add onsceen method.
CanvasRect: add onscreen method.
HUDBitmap: remove WeakPtr typedef which no longer needs to be public.
HUDElement: guard overlay-related methods with THIEF_USE_BROKEN_HUD_OVERLAY; rename fill_background to fill_overlay.

Script: don't trigger initialize method from an EndScript message, to avoid issues when loading saved games.
OSL: don't set self as HUD handler unless an element has registered, to allow coexistence with non-ThiefLib HUD scripts.

Numbered: add increment and decrement operators.

Interactive: use unsigned values for FrobAction.