Fix a endless loop of NSViewBoundsDidChangeNotification handler

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