Fix/searching relation picker

[sans-byte]

Fix for #7957
I have introduced debouncing so, after a user will stop typing for 300ms it will make a call

[FelixMalfait]

Hey, thanks a lot! But we don't want debouncing, it's actually nice to have realtime results as you type :))
What we want is to avoid the flickering effect, the previous result should remain visible as long as the new results aren't available (now we hide them when it's loading so it goes result -> white -> result -> white)