Switch WebP mime-type and alias around

The request for a WebP mime-type was made with the IANA, but still
not processed to this day. It's unlikely that the mime-type ever
be rejected, so add image/webp as the main mime-type.
Remove image/x-webp alias

It was only defined for a couple of weeks, and only within
shared-mime-info, no need to keep it.
Add application/x-riff mime-type

And make it steal audio/x-riff magic. RIFF is a container format,
not an audio format, so any type of data could be contained in RIFF.

See also http://en.wikipedia.org/wiki/Resource_Interchange_File_Format
Add JSON mime-type

As application/json.

https://bugs.freedesktop.org/show_bug.cgi?id=73646
Store MEDIA/SUBTYPE.xml files in lowercase

RFC 2045 says that MIME type names (e.g. text/plain) are
case-insensitive. Most are written by convention in lowercase, although
there are a few exceptions currently in the freedesktop.org.xml
database.

Store the separate mime files as lower-case to make them easily findable
(eg. the synonymous application/vnd.*macroEnabled* and *macroenabled*
mime-type should have the same filename).

https://bugs.freedesktop.org/show_bug.cgi?id=62473
Add *.jsm glob for Javascript

As used in Gecko/Firefox related projects.

https://bugs.freedesktop.org/show_bug.cgi?id=75031
Add magic and glob patterns for compressed x-spss-sav files

https://bugs.freedesktop.org/show_bug.cgi?id=71632
JavaScript and CSS are not subclasses of text/x-csrc

and add test files for CSS.

https://bugs.freedesktop.org/show_bug.cgi?id=73648
Remove *.CSSL glob for CSS files

It was probably meant to be *.CSS1 (one, not "L") which got badly
transposed when the KDE and GNOME databases were merged more than
10 years ago.

As nobody seems to have missed *.CSS1 and *.CSSL is wrong, remove it.

https://bugs.freedesktop.org/show_bug.cgi?id=76940
Add missing *.jsm test case file

Was missing from the earlier commit.
1.3