[Core] Adding fine-tuning options for the autocompletion

[rbayet]

Replaces #3467 (rebased with conflicts resolved)

[romainruaud]

So, I added 2 commits :

• the first one to ensure the "size of search terms to use", aka extension_size must be >0
• the second one to rename the extension_limit to extension_limited because it's a boolean which defines if the extension mechanism must be limited or not.