- Remove useless pointer back to DBusTimeout.
 - Fix memory error handling.
 - Fix race in accessing timer data.
 - Fix integer overflow.
 - Follow libdbus rules for retrieving time-out interval upon toggle.
 - Simplify expiration computations.

This fixes a race condition where the player was visible on the bus
with the MPRIS name, but did not implement MPRIS.

Note that we cannot hold the playlist lock while removing the callback,
since the callback function takes the playlist lock.

This is accessed with the playlock lock anyway.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>