gdesktopappinfo: Rank Keywords matches higher than GenericName

It makes sense to match on GenericName in case an application does
not provide any keywords, but the Keywords field has been added
to explicitly support the search case, while GenericName was used
to be displayed in menus, so it makes more sense to consider
Keywords more (or equally) relevant for search.

https://bugzilla.gnome.org/show_bug.cgi?id=711640