GCC will print a warning if the result of such function is ignored
This is useful for functions returning an allocated object (which
would leak if ignored), "getter" functions, and functions that are
"likely" to fail.

(Removed a unused macro)

Be carefull, it adds a new translated strings.

It fixes a regression introduced by fccf6aa0

It is provided for convenience for interfaces.

It may improve TS clock synchro algo.

Pointed-out-by: Sébastien Escudier

Crash happens if you do:
 - Play item
 - Activate vfilter
 - Stop item
 - Deactivate vfilter... BooM.
(cherry picked from commit c8764017)

(cherry picked from commit c31d99fdf67fbc9060fb9f413d06b0acf4efc1b6)

Already available in the interface sprefs.
This reverts commit c1a144df.