Allow non-local startup location

Only local directories were supported to be set as the starting location
of Dolphin. This commit fixes this by using an KIO::StatJob to determine
if the path is actually pointing to a directory or a file regardless if
it is a local or a remote path.

Additionaly a hard-coded exception for urls with the scheme "timeline"
was removed because with the new fix it is now obsolete.

BUG: 428885