Fix fictionbook loading

Prefer the backends that declare explicit support for a mime over the ones that get the support by inheritance
In the fictionbook case it was happening that application/x-fictionbook+xml inherits text plain
so when we ask for the fictionbook backend, we get both
the one that says
MimeType=application/x-fictionbook+xml;
and the one that says
MimeType=text/plain;
and then it happens that the second has bigger X-KDE-Priority

By prefering the ones that explicitely declare support for a mime we "fix" this

OTOH we may want to explore showing the backend selector if there's more than one by default

FIXED-IN: 4.11.0