Allow appending of text to any public.text file

This kind of shows off the power of UTIs and the changes we made in ß71.

To filter the entire 3rd pane for the 'Append To...' action to only show valid files, I just had to add

`indirectTypes = {public.text}`

to the .plist :)

The original aim of this was just to add .csv files, but I thought I'd go one step further.
We still need to check against the `textTypes` array in the code, since some files don't have proper UTIs set (like markdown files aren't public.text unless you install a markdown quicklook generator)