Make autocomplete minChars configurable

Due to CRM-6135 the jQuery autocomplete minChars setting in templates/CRM/common/Navigation.tpl was set to 1 to make the autocomplete mechanism usefull for Chinese language input, too.

For most other languages this makes the autocomplete too 'agressive' though, especially when you're dealing with slow typers, and causes extra database load without producing too meaningfull results.

In our local installation with ~40K (mostly German) contacts we've now changed the template to have minChars=3 (and are considering to maybe even go up to 4) as a search on 1 or 2 letters only does hardly ever produce any meaningful results anyway but consumes database and local browser resources and so slows things down without any extra gain for the user ...

Sure, we can just continue to have our custom Navigation.tpl, but IMHO an input language specific setting like this would justify a configuration setting of its own

(and if someone can point me at good docs on how to add a setting to the administratiton pages i'd even be happy to add this to the localisation admin page myself ...)