(Follow-up r75968) r75968 started extracting and displaying some svg metadata, but...

(Follow-up r75968) r75968 started extracting and displaying some svg metadata, but didn't add the messages it used.

It started to use the messages svg-description, svg-title, svg-width, and svg-height in the metadata table, without
defining those messages. I changed it to use the same messages as are used when displaying the same table on
the image description pages of jpeg files. ( exif-imagewidth and friends ). Personally I feel the metadata
should be about the image, not what format the metadata was stored in (a description of an image is still a
description of the image, regardless of the format of that image. JPEG images for example can have image descriptions in
3 different metadata formats [exif, iptc, xmp], it would be insane to make different messages for them). Furthermore,
I think we should rename all the exif-* messages to something like filemeta-property (but thats an issue for later).

I added one message, 'exif-objectname', since the svg title property didn't really correspond to anything in exif.
However, it does correspond very well to IPTC's object name property, which i hope we will start supporting in 1.18,
so I used that message for that.