When setting a default for the text/plain MIME association, it is not taken into
account for e.g. README and INSTALL files, and other text/x-* MIME types.

The problem is that in gnome_vfs_mime_get_default_application,
gnome_vfs_mime_get_default_desktop_entry is used, and returns the default only
for the current MIME type, not searching for its parents. When failing,
gnome_vfs_mime_get_all_desktop_entries is used and returns all entries,
including the parents'. Then the first one is used. Thus, you end up without
taking into account the default for the parents.

Proposed solution: fix gnome_vfs_mime_get_default_desktop_entry so that it
searches the parent MIME types for defaults.