Add destroy notify callback to prevent use-after-free crashes when
notifications are closed before their reposition timeout fires.

Fixes #236

Add checks for monitor_id to prevent crash during monitor change events.
Instead it falls back to last available monitor and allows notification
processing to continue without a monitor.

Fixes: #200

Recent versions of libwnck introduced a WnckHandler object to be used as
the main entry point into the library.

Returning an error for a dbus method is counted as handling a method invocation. Return true in the handler to avoid a use-after-free.