Merge the Layers and Objects views

Nobody has claimed this yet.

Name of merged view could be "Layers and Objects", as in Inkscape 1.2, or something more generic like "Hierarchy" or "Outline". Instead of Objects being a separate view, the Layers view will essentially gain an option (with toggle button) to show the objects.

Some notes:

* The Objects view has "Type", "ID" and "Position" columns that currently make little sense for Layers. Layers don't have a type (though that might change), and while they do have an ID, this is never actually relevant in Tiled (though object ID isn't really either). As position their offset could be shown.
* Mixed selection may have to be disallowed due to the different actions affecting objects and layers. In Inkscape, selecting a layer implicitly selects all its objects, but I'm not sure if that behavior is desired. One may want to change the current layer without changing which objects are selected (current behavior). We will need to differentiate "selected layers" from "layers actively selected in the layers view".
* Layers can be locked, but objects can't (though locking of objects has been requested, so that might change).
* Objects view has a filter, which of course should be available in the merged view.

Start by reviewing the existing Layers and Objects views, including their columns, selection behavior, locking, and filtering. Decide the merged view's name and interaction rules, then confirm that layers and objects can be managed together without losing the currently described behavior.

Written by the indexing model from the issue text.