GtkFileChooserDialog: Make it adaptive
It should remain as-is on desktops but it should work on phones too. We should decide how to handle the sidebar on mobile: should it be an overlay, or should it be more like a leaflet? Should the sidebar be visible by default on mobile or should it be the content? Or maybe we should have a completely different design on mobile?
There are many small elements to take into account too, like the path entries, the preview bar, or the elements which can be added to the action bar.