fix installation with pip if possible

[pavoljuhas]

icons are not installed to correct paths

[bobleesj]

@bobleesj the icons folder should be available in v3.1.0 since we use graft src in MANIFEST.in. Double-check this after re-cookiecutting and pypi release.

[bobleesj]

Resolved in diffpy_pdfgui-3.1.0rc4 with the new cookiecutter.

[bobleesj]

@sbillinge could you please close this? I checked manually from the sdist that icons are distributed.

[sbillinge]

Thanks @bobleesj