settings: Fix the Use Current Location button
During the Dolphin settings revamp (489b56b6?) this QUrl had been forgot to add back so that the FoldersTabsSettingsPage knows what is the current url.
This change checks for the main window of dolphin, then gets the URL of it whenever useCurrentLocation is called.
However, when this is used as KCM, the button is not created, since there it doesn't make sense.
(cherry picked from commit fc3a6594)
f8480aac settings: Fix the Use Current Location button, hide button in KCM
Co-authored-by: Akseli Lahtinen akselmo@akselmo.dev