Allow to remember view-properties for the search-mode

Per default Dolphin remembers the view-properties for each search-URL
individually. However when being in the search-mode, where the
search-URL is different each time by definition, it should also
be possible for the user to adjust the view-properties. The DolphinView
is not aware about any "search-mode", so an interface has been added
to provide a "context" for storing the view-properties. The context
will be adjusted by the DolphinViewContainer in case if the search-mode
is active.