Request to the medialibrary are done in the UI thread in the Qt interface

Hi,

All requests to the UI thread are done synchronously when creating the model or fetching its data in the UI.

It should be done in a medialibrary request dedicated thread so as to avoid blocking the UI while it's loading.

I'm currently doing some work to avoid exposing the medialibrary object directly and only expose the vlc_medialibrary_t in asynchronous callbacks. This would be the first step towards a fast and reactive UI, imitating what's done in the Android flavour with coroutines and IO dispatchers, but it would need additional work to provide as much information to the user as we can when things are loading.

At the same time, we should be able to split the number of elements and the actual data being loaded so that the user can have a quick feedback when the view change.