PERF: create unit menu on-demand for QLineEdit

[klauer]

Background

We noticed that a bunch of QMenu instances were in our typhos test suite at the top-level (QApplication.instance().topLevelWidgets()).
While tangential to the issue we were investigating in that repository, it seems unnecessary for PyDM to create these at __init__ time.

This PR

This PR creates the unit conversion menu on-demand.

Testing

• Test suite was updated
• Quickly checked the example to see if it still works:

[YektaY]

looks great!

[mattgibbs]

I think I remember the library that supplied the units for this being some big unwieldy thing that took a long time to import, and I suspect it effectively gets imported at launch time, because it lives in utilities. We should maybe move that import to happen the first time this menu is made…

[klauer]

I think I remember the library that supplied the units for this being some big unwieldy thing that took a long time to import, and I suspect it effectively gets imported at launch time, because it lives in utilities. We should maybe move that import to happen the first time this menu is made…

I think you might be remembering from a different project. PyDM uses scipy.constants (which for me, at least, imports pretty quickly). On our side, the library pint definitely does slow things down so we import on-demand as you've suggested.