The context menu for visualizer mappings should use "Select Visualizer" rather than "Show Visualizer"

[PathogenDavid]

This is something that came up when Gonçalo and I were discussing #1769

Rather than showing a visualizer directly, this submenu selects which visualizer is used by the mapping. The wording should be changed to reflect this.

Additionally changing this dropdown doesn't seem to be honored when the workflow is running (or maybe only when a data mashup is involved? -- should check) we should perhaps make it read-only to make this clearer.

[glopesdev]

@PathogenDavid Now that we fixed the behavior of #1841, I am slightly less inclined to go for this solution. Specifically, the bug with changing the context menu at runtime was fixed, and also clicking on the selected option will now always show/activate the visualizer (even if the selected option is already selected but the visualizer is closed).

Finally, the meaning of "Show" is valid even when the workflow is not running, as all visualizers marked to show will be immediately shown at workflow start time.

[glopesdev]

Could consider changing just for VisualizerMapping too and possibly also for the new ShowVisualizer node in #2137.