Replacing GLib macros.

Micro-optimization.

Merge duplicate symbol.

This was a memory leak, because "mime" was a std::string which created
another copy and discarded the allocated buffer.

As a side effect, this fixes a memory leak.

Remove the const_cast from HttpdClient.cxx, and avoid one allocation
in ClientRead.cxx.

The header glib_compat.h is now obsolete and can be removed.

Copy the according file from another project (i.e. XCSoar).  This will
allow copying more code more easily.

Try map_fs_to_utf8() first, and fall back to Path::ToUTF8() for
absolute paths.

Move code from map_fs_to_utf8().

Micro-optimization.

Consistently use std::string in LoadPlaylistFile().

Avoid the conversion to an allocated char*, let the caller decide.