See commit 71f0ed8b

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

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

Prepare to make it mutable.

Prepare to convert "partition" to a mutable pointer.

The "tagtypes" command now has several sub commands which can be used
to edit that mask.

Unused.

Referencing the attribute "partition" is illegal after Close(),
because Close() deletes "this".

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().