Thumbnailer: Fix potential use of dangling reference
Thumbnailer: Fix potential use of dangling reference

Event handler are unregistered when the eventManager copy gets
destroyed, however, we were destroying some captured by reference
variables in the event handlers, leading to a short window in which the
event could be invoked but would use a dangling reference

chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary