SingleScopeLens receives only the first global search

[Ivan Sagalaev]

A lens created with `SingleScopeLens` exposes a weird behaviour when viewed through the Dash home: it displays the results corresponding to the empty string which then don't change, no matter what's typed in the search box.

What exactly constitutes a bug here is a matter of what's intended by design.

Currently `self._lens` created in `SingleScopeLens.__init__` is hard-coded to not accept global searches. If it's the intention then the bug is that a similar property of `self._scope` is never assigned which defaults to True. The scope then accepts its first search right after being exported and before being added to the lens. And after being added to the lens the scope stops receiving "on-search-changed" signals and doesn't update its search results.

However the existence of the `global_search` method hints that those are, in fact, desirable :-). If yes, then the bug is that both `self._lens.props.search_in_global` and `self._scope.props.search_in_global` should be initialized from a corresponding property in `class Meta`.

I'm happy to make a patch for either case.