Provide language support for subtitles and spacers

[sanhozay]

Dummy checklist items are commonly used to create subtitles within checklists and for blank lines in the checklist dialog.

These can be created by defining dummy items in CLGen, but it is slightly cumbersome:

item("") {
    state("");
}

item("Subtitle:") {
    state("");
}

Some syntactic sugar could be added to the language to support these, e.g.

checklist("Climb") {
    check("Flaps", "UP");
    subtitle("Above 10,000ft:");
    check("Landing Lights", "OFF");
    check("Seat Belt Signs", "OFF");
}

These elements would create dummy elements in the items table without having to create them as dummy items in the .clg file.

[sanhozay]

Added in 1.2.0