Certain policy decisions should be in one place in the code (DRY). 

These could be based on preferences or environment (should we be using ttk?), which operating system we're using (what hand cursor looks best?) or even the version of the operating system we're running (do we need to include a sizegrip?), etc.

Similarly, there are some small utilities that should be available in many places (e.g. what events to bind to for context menu clicks, setting a dialog to modal appearance) that don't necessarily fit elsewhere.

I've attached ui.py which will be a starting place for these sorts of things.