bpy.types.NlaStrips needs to provide way to add Transition strips

At the moment, the only way to add Transition strips to the NLA editor is by using bpy.ops.nla.transition_add(). If you want to do use this from a script running in headless mode (or even if you are just running the script in a tab without the NLA editor open), it requires an elaborate work around where you have to search for the NLA editor screen area first, create a special context for it and then select the strips you want to link with the transition.

This makes adding a transition strip difficult, especially if you are running in headless mode. Using bpy.ops also requires you to manipulate the selection state of the environment, which is not desirable. It would be much better if transition strips could be created without requiring bpy.ops.

The bpy.types.NlaStrips.new() method currently only provides a way to add clip strips. It would be very useful to expand this so that transition, meta and sound strips could be created too.