Fix custom file crash and contrast not updating custom model

[StephenNneji]

Fixes #196
Fixes #193
Fixes #168

• Adding or deleting a custom file should now reflect in the contrast model dropdown

• The crash in issue Changing custom file causes crashing and is quite difficult #193 happened on this line

  RasCAL-2/rascal2/widgets/delegates.py

  Line 63 in 5d846d3

  func_names = self.widget.model.func_names[

  and was caused by how QT handled paths on windows. The file dialog path would be "c://file.m" but the ratapi path would be "c:/file.m" which led to a KeyError in the dictionary. I fixed by using as_posix to unify path style and used dict.get so None is returned if path is not found.

• I did some refactor so the full path of the custom file does not show up in the table anymore but in a tooltip when the user hovers over the filename cell, hopefully more readable?

[DrPaulSharp]

All works as expected.