Preflight tab - Edit: CheckListEditor.cs updated add preflight sub checklist item functionality and response time

[Matthew-Alti]

The following updates have been made to the Preflight ChecklistEditor form:

1. The controls' "+" button responds quicker
2. The controls' only display the "+" button next to each control item's last child item, and next to the main control item in the case when the control item does not have a child control item.
3. Bug fixes for when all items are removed from the form.