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.