Removed all signal-slot-connections related to DolphinNewFileMenu->errorMessage(QString)
in DolphinMainWindow and DolphinContextMenu and replaced it by a better solution.

Now we make use of the already existing DolphinNewFileMenuObserver singleton class to achieve a better
error handling, because every newly created DolphinContextMenu instance registers himself by DolphinNewFileMenuObserver
and we use this to connect the errorMessage(QString) signal of every DolphinContextMenu instance to the errorMessage(QString)
signal of the DolphinNewFileMenuObserver singleton class.

So we need only one connection from DolphinNewFileMenuObserver to DolphinMainWindow (or to DolphinPart) to
collect all error messages thrown by every DolphinNewFileMenu instance.

REVIEW: 112178