More events and general bugfixes

Added

• Added an OnDialogAdd, OnDialogSet, and OnClear events to RumorState
• Added the Clear command
• Added an OnWaitForAdvance and OnWaitForChoose event to Rumor
• Added a Cancelled property to Rumor

Changed

• AutoAdvance is now a time instead of a toggle, which indicates the amount of
  time before the Rumor will attempt to automatically advance
• Bindings are now stored in the Rumor instead of the Scope

Fixed

• Fix null variables not handled properly in expressions
• Fix no boolean literals parsing in RumorCompiler
• Fix nodes not checking for null values
• Fix If conditional not checking for null values