Move path creation to a separate class

We have to modify paths at various places in the application. A typical
thing that we want to spend extra care of is the handling of the root
directory. Move the modification of these to a separate class that we
can use everywhere.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Add option for going to the parent directory

It is possible to move down the directory hierarchy but we have no
option to go up the hierarchy. Add a menu item to the context menu for
doing this.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>