Adds custom file tab to project widget

[alexhroom]

This PR adds the Custom Files tab to the project widget. Fixes #45.

[StephenNneji]

Looks good so far

I think the filename should be stored as a relative path from the project directory, using an absolute path will make it so the project will not run if it is moved to another directory even though the custom file is in the same directory. Also the DSPC_custom_layers example crashes when I try to set the function because the path is stored as "examples/DSPC_custom_layers" whereas the path should be empty for a file in the same project directory

Also have an issue with the column size, out of the box the function name column clips the text

Also in edit mode it is possible to resize the columns so the button is not visible

I also cannot change the size of the name column directly, although the size can change if I change another column. Not sure if this is a bug or something intended but I don't think the behaviour is good as my first instinct is to reduce the size of the largest column then I find out I can't

[alexhroom]

@StephenNneji on the section resizes:

• your 'out of the box' looks quite different to mine - I don't have any clipping:
  
  and I'm not quite sure what to do about it, because as far as I can see the header view doesn't have an option to resize to fit the header text. I also can't reproduce your button shrinking bug (for me, the edit file column refuses to shrink smaller than the button widget!)

I'm also not quite sure how to make the name column resizable while also having it stretch to fill available space. This is the default behaviour of setStretchLastSection() for the last column but setting the resize policy of an arbitrary column to Stretch disables the interactivity.