matroska: Treat non-image structure as attachment and set mimetype

Otherwise each structure is named as GstTagImageInfo even if it does not contain any images which is misleading. Also set structure mimetype to fix data detection by mimetype in other plugins.

This is needed for e.g. to fix assrender fonts detection by file mimetype. As of now it does not work, so this MR comes in pair with gst-plugins-bad!1967 (closed).

This also introduces a structure named GstTagAttachmentInfo. For now unused, but once merged other plugins like assrender can adapt to check the structure name and ignore it if not matching name, thus speed up structures processing logic by not iterating over values in incompatible ones.