Fix a endless loop of NSViewBoundsDidChangeNotification handler

Steps to reproduce：
1. File - Open Recent 
Opens an existing markdown file,
2. Press the "plus" button in upper left corner.

Reason:
In function `-boundsDidChange:` which is an handler of `NSViewBoundsDidChangeNotification`:
`self.editor.frame = editorFrame;`
This will trigger `NSViewBoundsDidChangeNotification` again.