Add checkmarks in "Show legend" menu items
Add checkmarks in "Show legend" menu items

Check the menu item for the currently displayed legend type. When
hidden, both menu items are unchecked.

Closes #1146.

OpenChemistry/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,mathturtle/tomviz