Replace all remaining occurrences of boost::filesystem with std::filesystem.

Closes issue #6085 (closed)

While this merge request might look daunting for reviewers 90% of the changes are a trivial text replace and includes shuffling, but there are some files where more noticeable changes had to be made, these include:

• apps/mwiniimporter/importer.cpp
• apps/opens/model/doc/documentmanager.cpp
• apps/openmw/mwstate/character.cpp
• components/vfs/registerarchives.cpp

I have also added the following header components/misc/timeconvert.hpp that contains a simple templated function to convert between incompatible clocks, it's mostly a stopgap solution until C++20 becomes the default.