Fixes the "org.freedesktop.UDisks2.Error.AlreadyMounted" error.

Closes #485

Fixes assertion failure.

`light_song.Get()` could cause an assertion failure because the
`Manual<>` object must not be used if uninitialized.

Regression by commit ebc006ab

This fixes use-after-free bug in SimpleDatabase::Close(), accessing
the `root` object which was already freed by the `catch` block in
Open().

By having the Database on the stack first, we can avoid calling
Close() on the failed-to-open Database from Instance's destructor.

Closes #482

Make it less generic, to avoid clashes.

Closes GH-479.

Closes #477

Destruct automatically, even if leaving the scope due to exception
being thrown.

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().