Draft: qt: improve search performance by discarding intermediate requests

Currently, it does not seem that the old tasks are cancelled when the search pattern is modified. This seems quite bad because if the library is big then there will be a lot of stress in the medialibrary thread, possibly locking it for a long time.

This change causes crash on startup for some reason (it happens when I try to copy a vector of unique pointer to MLItem and feed it to callback functions), so I'm marking it as draft. I do not see if I did something wrong, so insights would be appreciated.