[mod] fix #903 : add a function is_accepted to engine.

[dalf]

fix #903
search.py calls this function before creating a thread for the engine.
if the function doesn't exist, the function is .... not called.

without this requests a useless thread is created for the dictzone and currency engines.

[dalf]

Another more simple implementation : just define a regex for an engine.
If search.py see a defined regex, it checks if the query matches this regex.

dictzone and translated do extended check of the languages in is_accepted, with the regex version it won't be possible.

[dalf]

Supersed by #2159