The list of partitions is cleared automatically.

This replaces the old `dead` flag which was unreliable; it was `false`
if the EventThread was not yet started, which could cause deadlocks in
BlockingCall().

They will be stopped by ~EventThread() when the `Instance` is deleted.

 * "lsinfo" argument is optional
 * "tagtypes disable" arguments are mandatory (typo)

Currently it falls back to system default device (either internal speaker or headphone) when device not found. 
I believe it is a better to fail in this case, to make it better aligned with platforms (such as alsa).

Previously, a seek to the end of the file would cause an assertion
failure in SparseMap::Check() because the given offset was invalid.

Closes #453

Must lock the mutex before calling any of the unprotected InputStream methods.

Closes #462

Fixes build failure after commit
0cea67ee

GCC 9's libstdc++ is unable to use forward-declared types as
std::vector item because the compiler wants to resolve `noexcept()` on
the item destructor.