Change notification callback and handle_events types

Change notification callback and handle_events types

In earlier implementation (commit db775397) single event was passed to
to application notification callback as a string type and if there was
multiple notification then as a list of string. Similarly
'handle_events' function was either returning a single event as string
or a list of string. In a code it is unpleasant to handle variables what
can be either string or list type, because in application you need to
check always the variable type what comes from SDL for these functions
and based on the type either handle the value as a string or list of
string. That's why by this commit change event(s) to be passed as a list
to the notification callback what application has registered and also
now 'handle_events' function returns always received notifications a
list. Because of type change this code change is not backward
compatible.

Issue-ID: RIC-795

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I49f2bd1012d93f184945c891df03ed83ed83d544