Show template details (parameters, concepts, specialisations) for template types

[whisperity]

Currently, for a template class, the only way to figure out what arguments are given for said template is via inspecting each Usage. (The Usage option in the Info Tree only lists template_class, without the actual template arguments.)

In general, for template classes and methods, the following should be added to the InfoTree:

• List of template parameters, potentially with their defaults (and later, the concept constraints) marked
• List of explicit template specializations (this can be important, think of std::vector<bool>)
• List of other template specializations at instantiations (not necessarily the standard-defined "template instatiation location", but just in general that TU a.cpp uses std::map<std::string, int>, for example).