Move utils.get_standard_dir to its own file.

This is a preparation for #19 and #20 because there are too many functions
related to standarddir in utils.
Small whitespace fix.
Fix removing of eventfilter on shutdown if not present.
Add an init method for standarddir.

This is another preparation for #19 and #20.
Create standard dirs correctly. Fixes #20.
Add CACHEDIR.TAG to cache. Closes #19.
Fix docstring.
Remove checks if standard dirs exist from unittests.