Rethink file picker fallback

The initial FilePicker implementation in Oxide tries to open a ContentPeerPicker and then falls back to a standard FileDialog if that fails. The problem with this is that webbrowser-app depends on content-hub, so opening a ContentPeerPicker will always succeed in any application, even in scenarios where it displays no useful peers and the application could open a standard file dialog.

The way I think this could work is:

- If ContentPeerModel indicates there are no peers (or the only peer is the current app) AND application confinement doesn't prevent the application from browsing the home directory, open a FileDialog, else open the ContentPeerPicker.
- When displaying the ContentPeerPicker, if application confinement doesn't prevent the application from browsing the home directory, display a button at the bottom of the window to switch to a FileDialog.