qt: improve ml playlist insertion perceived reactivity

As we have the element content when inserting them in the library, so we can
insert them "a priori" in our cache and update the interface before the
medialibrary notifies us with the actual content ("a posteriori"). If assumption
was wrong, diffutil will correct it

modules/gui/qt/medialibrary/mlplaylistmodel.cpp

@@ -59,9 +59,12 @@
 
     setTransactionPending(true);
 
-    m_mediaLib->runOnMLThread(this,
+    struct Ctx {
+        std::vector<std::unique_ptr<MLItem>> medias;
+    };
+    m_mediaLib->runOnMLThread<Ctx>(this,
     //ML thread
-    [medias, id, at](vlc_medialibrary_t* ml) {
+    [medias, id, at](vlc_medialibrary_t* ml, Ctx& ctx) {
         std::vector<int64_t> mediaIdList;
         for (const auto& media : medias)
         {
@@ -77,8 +80,8 @@
                 if (ml_media == nullptr)
                     continue;
             }
-
             mediaIdList.push_back(ml_media->i_id);
+            ctx.medias.emplace_back(std::make_unique<MLPlaylistMedia>(ml_media));
             vlc_ml_media_release(ml_media);
         }
         if (mediaIdList.size() == 0)
@@ -87,7 +90,8 @@
         vlc_ml_playlist_insert(ml, id, mediaIdList.data(), mediaIdList.size(), at);
     },
     //UI thread
-    [this]() {
+    [this, at](quint64, Ctx& ctx) {
+        insertItemListInCache(std::move(ctx.medias), at);
         m_need_reset = true;
         endTransaction();
     });