fix for disable_linting appending without check

[vindolin]

1. When running the disable_linting command multiple times, filename or
   window_view gets appended to the
   ANACONDA['DISABLED']/ANACONDA['DISABLED_BUFFERS'] even if it's already
   there.
   The user then has to run the enable_linting command the same number of
   times to enable it again.

2. is_enabled looked for view.id() instead of (self.window.id(), view.id()).

Also I made the two files look more unified.

[DamnWidget]

Why not just use a set instead of a list? We will end with the same result but in a much cleaner way.
What do you think?

[vindolin]

That was my first idea but ANACONDA['DISABLED'] is a PersistentList.

[DamnWidget]

Yeah that's true then what do you think of moving it to the PersistentList itself into the append method (just call the append method in the super class if the element is not in the list already) and make 'DISABLED_BUFFERS a set?

[vindolin]

I look into this when I'm back from a vacation in about a week.