Adapt to Orca 47

Adapt to Orca 47

The screen reader Orca has seen some fundamental changes between
Orca 46 and Orca 47. While they are improvements overall, they do
require changes to Dolphin to preserve the intended user
experience for Orca users.

The biggest change is perhaps that Orca will now not only announce
changes to the currently focused item, but also of its parent,
which means we do not need to pass focus around between file items
and the main view within Dolphin, but can keep focus on the file
items most of the time. This commit implements this.

The only exception of when we cannot have focus on the items within
the main view is when the current location is empty or not loaded
yet. Only then is the focus moved to the view itself and the
placeholderMessage is announced.

This commit worsens the UX for users of Orca 46 or older, so this
should only be merged once most users are on Orca 47 or later.