Add support for application/vnd.rar, fixes opening RAR with shared-mime-info 1.7

shared-mime-info 1.7 introduced the official IANA mimetype
application/vnd.rar and degraded application/x-rar to alias.

For now we need to register both mimetypes. Once distributions start
shipping shared-mime-info 1.7, we can drop the alias.

For this reason, we cannot have a reliable test case in mimetypetest.
Also, in PluginLoader::sortByComment() we need to check whether
mimetypes are valid, because application/vnd.rar might not be in
QMimeDatabase().

BUG: 368786
FIXED-IN: 16.08.2

Closes T3729