KPluginMetaData: add initialPreference() getter

Summary:
useful to sort parts by preference.

auto byInitialPreference = [](const KPluginMetaData &left, const KPluginMetaData &right) { return left.initialPreference() > right.initialPreference(); };
std::sort(plugins.begin(), plugins.end(), byInitialPreference);

Test Plan: gvpart.so now comes first among all parts that can handle a jpeg file

Reviewers: arichardson

Maniphest Tasks: T12340, T12173

Differential Revision: https://phabricator.kde.org/D26443