Don't re-open already-open URLs when using session-restore feature

If Dolphin would be asked to open a location that is already open, don't open it again in a new tab; instead switch to that view. Supports tabs and split view.

This situation became common after !75 (merged) was merged, and we have gotten complaints about the feature because of it. This MR does not revert the behavior of restoring old locations before opening the user-specified new one, but rather refines the behavior to improve the somewhat common use case of opening a location that was already open last time Dolphin exited.

If need be, we can still revert the feature in the future, or make it optional to restore previous locations when launched with URLs.

cc @broulik