Add input component to choose math mode for inner nodes and leaf nodes

[dharasim]

Examples:

• no math [.I [.V G7 ] [.I C ] ]
• math for inner nodes [.$I$ [.$V$ G7 ] [.$I$ C ] ]
• math for leaf nodes [.I [.V $G7$ ] [.I $C$ ] ]
• math for inner and for leaf nodes [.$I$ [.$V$ $G7$ ] [.$I$ $C$ ] ]

The choice could be implemented using checkboxes.