Instead of writing QVector<Plugin::Descriptor> everywhere, mix up
iterators etc. centrally define Plugin::DescriptorList and use it
everywhere.
Additionally made pointers to Plugin::Descriptor and
Plugin::Descriptor::SubPluginFeatures::Key const in various places
so we can use ConstIterator everywhere when iterating through
Plugin::DescriptorList.
(cherry picked from commit 2287eca802)