Fix flickering during MathJax setup

When MathJax is on, delay window refreshing until it's done rendering.
This is implemented by adding a WebScripting object to the preview, and
register a MathJax signal handler that invokes the window refreshing
code via the WebScripting object.
Merge branch 'mathjax-flicker'

Conflicts:
	MacDown/Code/Document/MPDocument.m
Trigger UI updates on reload button pressed

The rendering methods are called, but the UI does not update. Maybe due
to caching? Need to investigate.
Refactor notification handler to separate methods
Merge branch 'fix_116-reload-refresh'

Fix #116