PartLoader: move UI strings out of template code in header

Fixes QObject::tr() text not being extracted currently, as well as
i18n call being done with the catalog/tanslation domain of the
consumer call, not the kparts one.

Removing the i18n call and the KLocalizedString include from the public
header also fixes the need to have KI18n in the public link interface
(also missing ATM).