Hide visible formatting

[GoogleCodeExporter]

At the moment, glasscat seems to be hardcoded to show visible tabs and 
spaces--I can is a very handy feature in programming whitespace-sensitive 
languages like python, but when using Glasscat as primarily a text file viewer 
and light editor (I have glasscat configured as the default text editor on my 
desktop), it gets in the way and can make reading harder. 

Both of the other available preferences (Right margin and word wrap) seem to 
pertain to swapping between text and programming, so it doesn't seem 
unprecedented to make this configurable

Original issue reported on code.google.com by [email protected] on 28 Sep 2010 at 8:15