Eliminate yet another access to the global ConfigData instance.

`AllocatedPath::FromUTF8Throw()` will never return a nulled instance.

Modeled after std::filesystem::operator/() from C++17.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

default_value defaults to nullptr.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Previously, the pointer was moved to undefined memory.

Increment the "path" earlier.

This was building broken paths like "/home/foo/~/bar".  Bug found by
Maarten de Vries.

Use Path::FromUTF8() for the g_get_home_dir() return value instead of
assuming it's already FS charset.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.

Better error messages.