Resolves https://gitlab.com/OpenMW/openmw/-/issues/8100
Also removes some old crud.
Hopefully the old crud is all:
* Handled automatically by CMake now we're using the modern approach.
* A hack-fix for a problem caused by not using the modern approach.
* Massively outdated so no longer necessary.
If it turns out this makes CI fail, I'll tweak things as necessary.
Changes that might not be wanted include:
* Getting rid of our BOOST_STATIC CMake option. In cases where the CMake config doesn't make the one correct choice from the build environment (i.e. because there's a choice) the CMake config exposes the option already.
However, we were forcing this on for Windows, so that might matter.
It seems to default to static on my machine even though I thought I read something suggesting otherwise, so we'll see how things go with that.
If we eventually put CMake in charge of installing dependency DLLs this will be a moot point as we won't need to care.
* Bumping the minimum version of Boost to 1.70.0, as that's the first with working CMake config.
It's from 2019, so plausibly there are distros too scared to use a library from five years ago as it can't legally drink in the US (although it could in limited quantities with parental supervision in the UK, as long as it's just something inconsequential like a single sip of beer).
I tried to fix https://gitlab.com/OpenMW/openmw/-/issues/8080 by making it so that instead of crashing, we showed an error.
In doing so, I discovered some problems with plugin sorting and the refresh button, like:
* it forgetting the non-user content files somewhere
* nothing guaranteeing that built-in content files stay at the top of the list and them only being there because the first data directory that provides them is usually the first data directory
* it forgetting the non-user content files somewhere else
* it looking like it'd forget any kind of non-user setting under certain circumstances
I fixed those problems too
To avoid implicit conversion via Normalized which creates NormalizedView from a
temporary Normalized. Mark constructors explicit on purpose so there is no
ambiguity on implicit conversion when there is an overloaded function like:
void f(const Normalized&);
void f(NormalizedView);
