Widget issues in qt 5.12

**Behavior**: In a node widget menu, making a selection on one widget sometimes causes the size of that widget to change. However, in my Qt 5.12 build, the widget menu isn't always resizing the "space" for the widget in reaction to that change. Widget updates are also happening very slowly.

Based on my testing, the widget space is resized correctly if any of the following are true:
- The widget gets _smaller_ after the change (e.g., going from a mode with a slider to a mode with no slider).
- The widget gets bigger and the widget menu has a scroll bar or will need one after the change.

@aganders3 or @grlee77, do either of you have any idea what might be causing this? Seems like a GPI-Qt interaction bug, perhaps? I'm happy to look into it as a good bug fix to get some more exposure to the GPI framework code, but thought you might have a better idea of where to start.

**Environment**: I am only seeing this on OSX, not Ubuntu. Installing `gpi_core` into a new environment should give you pyqt 5.12 to try and replicate. I've seen this with Python 3.7 and 3.8 both so far so I don't think the python version matters.

Example for switching from `pass` to `B/E` in a Reduce node.