Premiere Pro: Add ApplicationEventHandler type definition and new event

[galaddirie]

Updates

• Created ApplicationEventHandler type to specify the shape of event handlers for each event, from Adobe CEP samples
• Updated app.bind method signature to use the new type
• Added "onItemsAddedToProjectSuccess" ApplicationEvent, defined in Adobe CEP samples, triggers on item add

[galaddirie]

fixes #131

[galaddirie]

Also updated the app.unbind eventName to use ApplicationEvent