Path Filleting: present dialog to select radius

Previous filleting radius was hardcoded to 5 meter but
now a dialog will be presented to allow changing this.

Unfortunately because the dialog should have a handle to
the parent widget this requires adding an optional parameter
to the processPath call which seems unclean.  Really the UI
handling should be separated.
Path algorithm: create a configure method

This fixes the problem with the previous fix where the UI code was
being called from the algorithm.  Now there is a separate configure
method for the IPathAlgorithm which takes in the QWidget for the
calling UI and can present whatever dialogs are needed.
Path filleting: ability to unfillet a path

Now the path is saved when the fillet button is clicked.  Then
the unfillet button will restore the previous path after a succesful
filleting.