Assume undefined icons to be None (GTK)

[volxage]

Instead of just checking if the row has an icon, the new proposed conditional checks if the "icon" attribute is a member of the row at all, then checks if the "icon" attribute is None. The code is also slightly refactored as if not hasattr... is not as readable (to me) as getattr(...) is not None

Previously, without setting a row's icon either explicitly, an error was thrown because getattr(row, "icon") is looking for an attribute that wasn't provided. After this change, when rows are defined without the icon attribute, the rows render without an icon and the code doesn't throw an error. This is only for GTK's implementation. (My first ever pull request, I wanted to start small :)

PR Checklist:

• All new features have been tested
  (NA) All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Thanks for the PR!

CI is currently failing because the PR is missing a change note.

In terms of the code itself - our general code style is to avoid hasattr() in favor of try/except AttributeError blocks.

I also want to flag that I've just started an audit of the DetailedList widget - see #2011 and #2017 for analogous audits of Table and Tree. These tests include checks of "has icon", "has no icon", and "has empty icon", which will should catch issues like this one.