Preparations for FileChooser implementation (Part 1)

António Fernandesrequested to merge
wip/antoniof/filechooser-prep-part-1 into main

This mostly deals with preparing NautilusWindowSlot, NautilusFilesView, and other components to be reusable in a window which is not a NautilusWindow instance.

Overview in #3402 (closed)

Part of #3401 (closed)

Edited by António Fernandes

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