Cleanup: Simplify open document

[jeremypw]

• Move creation of Document objects into DocumentView and change first parameter to string (path)
• Only create new objects when needed
• Update calls to this function appropriately
• Lose unneeded intermediate functions in MainWindow and call the Document View function directly
• Simplify the open_document function by DRYing and treating all documents the same regarding focus, cursor_position and range, whether newly created or existing .