Implement view mode change when selecting mode form menu 'view'. Clean up code a little
On branch master, changes to be committed:
	modified:   mainwindow.py
Implement markdown note editor with syntax highlighting.
On branch master, changes to be committed:
	modified:   mainwindow.py
	new file:   mdhighlighter.py
	new file:   noteeditor.py
	modified:   quark.py