Migrate lal_trigger element from gstlal-burst to gstlal

This puts the "generic" trigger generator (lal_trigger) in a location that's more suitable for use in both gstlal-burst and gstlal-inspiral as needed.

As it stood before, making use of this element within gstlal-inspiral is problematic since gstlal-inspiral would need to depend on gstlal-burst, but gstlal-burst implicitly depends on gstlal-inspiral.

All of the changes in various files are just to update the relevant packaging so it all checks out. I confirmed that the element is registered in the right place now, and everything that depends on this generic trigger generator still works as expected.