Don't session-restore invalid paths

When session restore is populating the main window, it's unconditional;
you'll get whatever was there before. This can be a problem if any of
those things are now missing. For example, maybe you were browsing files
on a removable disk, then quit Dolphin, and finally removed the disk. The
next time you launch Dolphin again, it will try to show you the
view from the now-missing removable disk.

To prevent this, we now look at all the URLs in all of the view
containers that were created after session-restore has finished doing
its thing; if any of them are invalid local URLs, we change the URL to
the home folder instead to avoid showing the user a view with an invalid
location in it.

BUG: 427619
FIXED-IN: 20.12