Cycle widgets now respects selectable widgets

[voryzen]

• Added 'selectable' boolean, in the case that user wants to include cycling over non selectable widgets.
• function creates a dict of 'selectable' widgets, mimicing the default self.get_widgets() function
• keeps as much of the default code as possible

Quick summary of pull request...

• [x] I have read the contribution guidelines
• [x] CI Unit tests pass
• [x] New functions/classes have consistent docstrings

What this pull request changes

• Cycles widgets now respects the 'selectable' widgets in the widget set

Issues fixed with this pull request

• #170

Optionally, what changes should join this PR

• Open to any changes, if required ...

[voryzen]

@jwlodek are there any changes, you'd like to see in this bugfix?