implement for the "load" command the same logic used for the "add"
command: local clients can load playlist specified as absolute paths.

For relative paths the old logic is preserved: first look for a stored
playlist, then look in the music directory.

Even if the LocalStorage is available, return the "udisks://" URI when
the MapUTF8() parameter is an empty string.  This fixes the mount URI
in the state file.

Fixes a memory leak, or an assertion failure in the debug build.

Closes #487

Closes #471

Similar to commit ff1ff1e5

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.