Revert "separate all mime types with a colon"

This reverts commit 74813a4c07c3b36565493bebc006fcd0d5e62aa9.
Fix MIME type lists in desktop files

Make sure we always use a ; betweeh mime types and a trailing ; as well
in desktop and backend files.

https://bugzilla.gnome.org/show_bug.cgi?id=732216