Fix build with libc++ 22 - #665

Building with libc++ 22 fails:

sift_down.h:49:39: error: no matching function for call to object of type 'LocaleBasedSorting'
comparators.h:69:7: note: candidate template ignored: could not match 'std::pair' against 'operator_brackets_proxy'

libc++ 22 calls the comparator with it[n] instead of *it. For Boost's transform iterator, it[n] is a proxy object, not the element itself. The std::pair and RunnableItem overloads of LocaleBasedSorting are templates, and templates do not convert a proxy to the element type.

Each of the two templates is used with a single type, so this replaces them with plain overloads for those types. Plain overloads convert the proxy, as the existing std::string, Playlist and Song ones already do.

Fixes #663