Part: Introduce PreviewUpdateScheduler

This commit introduces PreviewUpdateScheduler class that is responsible
to schedule the true recompute of the preview. View Providers (or other
components) can use this service to ask for the preview recompute to
happend at a time that is convinent for a program and that won't impact
performance.

The provided implementation uses Queued Connections in Qt to calculate
preview essentially on next run of the event loop. It allows business
logic in FreeCAD (like property propagation) to execute fully and then
recompute preview once. This greately reduces number of recompute calls
for previews.

src/App/DocumentObserver.cpp

@@ -817,6 +817,9 @@ App::DocumentObject* DocumentObjectWeakPtrT::_get() const noexcept
     return d->get();
 }
 
+DocumentObjectWeakPtrT::DocumentObjectWeakPtrT(DocumentObjectWeakPtrT&&) = default;
+DocumentObjectWeakPtrT& DocumentObjectWeakPtrT::operator=(DocumentObjectWeakPtrT&&) = default;
+
 void DocumentObjectWeakPtrT::reset()
 {
     d->reset();