Prevent unnecessary reloading of KDirLister on startup

When Dolphin is started with a directory as argument unnecessary reload operations of KDirListers are done. The patch improves this by just letting the DolphinView internally do a loading as soon as it gets visible (and without reloading). Also in case if only one directory is passed as argument the current tab gets reused instead of creating new tabs and removing the current tab.

Thanks a lot to David Faure for pointing out this inefficient handling.