Plug a memory leak when reading misnamed tag files

Don't leak the file name if we can't determine to which filetype it
belongs.  Thanks to Pavel Roschin for spotting this.

Also, remove an useless second function indirection that simply made
the code harder to understand.  This will make Matthew happy ;)
Fix mingw-gcc 'ordered comparison of pointer with integer zero' warning
Update of Slovenian translation by Jože Klepec
Update notes for adding a custom filetype
Improve custom filetype formatting/links
Fix missing warning section