This change is required to support copies of the same cast member running in different channels.
* DIRECTOR: Move widget ownership from cast member to channel
* DIRECTOR: Cache cast dimensions from createWidget
* DIRECTOR: Add null checks for createWidget
* GRAPHICS: MACGUI: Clear active/hover WM references if the widget is destroyed
* DIRECTOR: LINGO: Re-add null check for kTheEditableText
* DIRECTOR: Avoid destroying widgets if the channel isn't dirty
* DIRECTOR: Fix dirty test
* DIRECTOR: Fix text field writeback
* DIRECTOR: Fix null cast check
* DIRECTOR: Add bbox as argument to createWidget
* DIRECTOR: Add setCast method to Channel
* DIRECTOR: Fix use-after-free in Channel::getMask
* DIRECTOR: Fix bounding box for Button sprites
* DIRECTOR: Cache widget dimensions in cast member
* DIRECTOR: Remove widget from channel if nextSprite is empty
* GRAPHICS: MACGUI: Fix MacText::splitString for empty string case
* DIRECTOR: Add null checks for movie and score in processEvents
