Replaces GLib's g_str_has_prefix().

If fs_charset is empty, i.e. we're using the default "utf-8",
GetFSCharset() should return exactly that instead of an empty
std::string.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Move to TagSettings.c and use C99 initializers.

Replaces GLib's GError.