Consider making VisualizerType selection in VisualizerMapping a property grid dropdown.

[banchan86]

Currently specifying the VisualizerType property in VisualizerMapping is done by right clicking the operator and clicking Show Visualizer in the context menu. That seems a little unintuitive to me because of a few reasons:

1. I usually associate Show Visualizer with selecting a visualizer for operators that I double click on for visualizers, while I wouldn't double click the VisualizerMapping.

2. The VisualizerMapping operator I see as closer to say the PropertyMapping operator where I would go to the property grid to select/map values.

3. The presence of the property in the property grid suggests that I can change values there (even though I know it's not greyed out and not editable).

In addition, I propose making an explicit VisualizerType selection when the operator is first connected to an input operator. I am not sure what the rules are right now, I am guessing that if no VisualizerType is selected, the VisualizerMapping defaults implicitly to the first value in the Show Visualizer context menu without an indication that it has been selected. That kind of hinders the discoverability of the VisualizerType selection in VisualizerMapping and I only discovered it when I was trying to debug an issue.

[PathogenDavid]

Related: #1791