Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

This commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Empty currently, but code will be added song.

Manipulates the playback range of a queued song.

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.