Halloween Update

New Halloween-themed Scene, and Templates. (Booo!)
App identifies and warns the user when the imported 3D object is not watertight and can’t be edited.
Fixes around Undo and Vertex painting related crashes.
Ability to abort or cancel the import process of a 3D file. (Ongoing development)
Updated progress bar: App shares certain information about the import process.
App warns the user if the import process is expected to take long.
Minor rendering related bug fixed.