eina: Introduce Eina_Slstr for short-lived strings

eina: Introduce Eina_Slstr for short-lived strings

Built on top of the new 'postponed' free queue, the short-lived
strings API allows users to return new strings without caring
about freeing them. EFL main loop will do this automatically for
them you at a later point in time (at the end of an iteration).

The APIs provided will either duplicate (copy) or more generally
steal an existing string (char *, stringshare, tmpstr, strbuf),
taking ownership of it and controling its lifetime. Those strings
can then be safely returned by an API. From a user point of view,
those strings must be considered like simple const char *, ie.
no need to free() them and their validity is limited to the
local scope.

There is no function to remove such a string from the freeq.

The short lived strings API is not thread-safe: do not send a
short-lived object from one thread to another.

@feature