If Dolphin crashes, changes that have been made to settings are often
lost. This has been a minor annoyance for me when developing, but it has
become a much bigger issue recently due to the problem where Dolphin
freezes on shutdown for ROG Ally users.

Instead of saving the config when certain arbitrary settings are
changed, let's save the config when the user closes the settings window.

Settings: Open file dialogs at user path instead of custom path

When opening a file dialog to set the location of a custom path, use the
corresponding user path as the starting location instead of the current
custom path.

When no custom path was set the dialog would be opened with a blank path
which causes Windows (not sure about other platforms) to open the dialog
at the same location where the last dialog was closed, or at the current
working directory if no previous dialog had been opened.

If a nonempty custom path has been set then it has also set the
corresponding user path, so the behavior in that case is unchanged.