Portable Launcher (plus a whole slew of bugs fixes for problems I found that I suspect aren't on the tracker)
Closes#6846
See merge request OpenMW/openmw!3925
Previously it was quasi-mandatory - lots of things would add it, e.g. when running openmw through the CS, but it could technically be disabled.
Now it's treated like the resources/vfs directory and implicitly added by the engine etc.
It doesn't work yet due to osgDB::listAllAvailablePlugins returning a list of paths to dynamic libraries.
That means:
* the check fails when the required plugin is linked statically.
* we're going to have to do something to slice up the filenames.
* there'll probably be unicode errors when the OpenMW installation path isn't representable by the current eight-bit code page on Windows.
Alternatively, we can switch to listing the required file extension support, and use osgDB::Registry::instance()->getReaderWriterList() and each element's supportedExtensions() function, but I don't think we've actually got that list of extensions anywhere and it might get desynced with the existing list of plugins if we add more.
Merge conflicts included:
* One setting being removed (branch had changed its type).
* One setting's description being changed (branch had changed its type).
* List of files in components/files was changed both upstream and on the
branch.
* Upstream had changed something in a file the branch deletes.
Tests which no longer work are commented out.
Some of these don't work because they're effectively testing for the
presence of bugs in the old implementation.
Others don't work because we're no longer accidentally disabling the
boost::program_options feature where it generates an error if only part
of a token gets consumed.
These will be fixed by later commits.
These would only take their value from the highest priority source, so
specifying `openmw --content anExtraEsp.esp` would override all the
content files in the user cfg file, and the user cfg file would override
any in the global/local one.
